"Acerentomon, a very primitive insect. H, Head; TH.1, TH.2, TH.3, terga of thoracic segments; 1, 2, 3, the thoracic legs; A.1, A.2, A.3, A.4, abdominal appendages; P.A., eighth abdominal tergum; G., genital aperture; AP.P., post-anal appendix." -Thomson, 1916

Acerentomon

"Acerentomon, a very primitive insect. H, Head; TH.1, TH.2, TH.3, terga of thoracic segments; 1, 2,…

The name of animals of the class Arachnida. Scorpions have an elongated body, suddenly terminated by a long slender tail formed of six joints, the last of which terminates in an arcuated and very acute sting, which effuses a venomous liquid.

Arachnid Scorpion

The name of animals of the class Arachnida. Scorpions have an elongated body, suddenly terminated by…

Scene showing various animals of the Division <em>Articulata</em>.

Articulata

Scene showing various animals of the Division Articulata.

The stages of a butterfly: the caterpillar, the pupa, and the adult butterfly.

Butterfly

The stages of a butterfly: the caterpillar, the pupa, and the adult butterfly.

A Thysanuran, the Campodea.

Campodea

A Thysanuran, the Campodea.

A centipede.

Centipede

A centipede.

The House Centipede (Scutigera coleoptrata) is a yellow and gray centipede with thirty legs. It was once referred to as Cermatia forceps.

House Centipede

The House Centipede (Scutigera coleoptrata) is a yellow and gray centipede with thirty legs. It was…

A female cockroach, Periplaneta orientalis.

Female Cockroach

A female cockroach, Periplaneta orientalis.

A male cockroach, Periplaneta orientalis.

Male Cockroach

A male cockroach, Periplaneta orientalis.

The arthropod, the freshwater crayfish.

Crayfish

The arthropod, the freshwater crayfish.

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacean, and is hence distantly related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages. Barnacles are encrusters, attaching themselves permanently to a hard substrate. Shown is a barnacle larva.

Barnacles

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacean, and…

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacean, and is hence distantly related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have two nektonic larval stages. Barnacles are encrusters, attaching themselves permanently to a hard substrate. Shown is a developed adult barnacle.

Barnacles

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacean, and…

"Bilaterally symmetrical bodies composed of a series of rings or segments, each of which bears a pair of jointed appendages, or limbs; a well developed and symmetrical nervous system of many connected ganglia. The lower arthropoda are the crustaceans, such as the lobster." &mdash; Hinman, 1888

Lobster

"Bilaterally symmetrical bodies composed of a series of rings or segments, each of which bears a pair…

A millipede.

Millipede

A millipede.

A mosquito.

Mosquito

A mosquito.

"Diagram of structure of Scopionidae (most of the appendages removed). IV. to XX., fourth to twentieth somite; IV., basis of the pedipalpi or great claws; V., VI., of two succeeding cephalic segments; T, telson of sting; a, mouth; b, alimentary canal; c, anus; d, heart; e, a pulmonary sac; f, line of ventral ganglionated cord; g, cerebroganglia." -Whitney, 1911

Scorpion Diagram

"Diagram of structure of Scopionidae (most of the appendages removed). IV. to XX., fourth to twentieth…

The dorsal view of Eurypterus remipes, an extinct sea scorpion from the Silurian period.

Dorsal View of Sea Scorpion

The dorsal view of Eurypterus remipes, an extinct sea scorpion from the Silurian period.

Pantopoda, or Sea Spiders, are a remarkable group of Arthropod animals, perhaps intermediate between Crustaceans and Arachnids. They are spider-like organisms, having four well-developed walking legs, often of great length. The body consists of an unsegmented proboscis, a trunk of four segments, and an unsegmented abdomen. There are no gills -- a point of distinction from the crustaceans; but the number of appendages in front of the legs is different from that of the true spiders. In the more typical forms there are three appendages in front of the legs -- first a small pair of chelate limbs, then two pairs of slender appendages. The first of these are used like the chelae in food getting; and the second seem to be functionless in the female, while in the male they carry the eggs. In the common shore sea spider <i> (Pycnogonum littorale) </i> , however, the two anterior appendages are absent in both sexes, and the third or egg-carrying pair are present only in the male. This form is found beneath stones, and is of a yellowish color, with thick, knobbed legs. Another common form is <i> Phoxichilidium femoratum </i> , which is of a pink color, has very long legs, and is commonly found on bright-colored weeds in shore pools. A representative American species is <i> Phoxichilidium maxillare </i> , which spreads about an inch and varies in color from deep brown to almost white. It is commonly found in shore pools; and its young live for a time withing hydroids. It is in the ocean abysses that the sea spiders reach their maximum size. One deep-sea form <i> (Colossendeis gigas) </i> has a span of nearly two feet from tip to tip of the legs. There is a metamorphosis in development in the sea spiders, the larvae having at first three pairs of appendages.

Sea Spider

Pantopoda, or Sea Spiders, are a remarkable group of Arthropod animals, perhaps intermediate between…

A spider.

Spider

A spider.

"Anurida maritima, one of the primitive wingless Collembola." -Thomson, 1916

Springtail

"Anurida maritima, one of the primitive wingless Collembola." -Thomson, 1916