THOEACOSTBACA. 461 



The Thoracostraca, like the Arthrostraca, possess a cephalo-thorax 

 composed of thirteen segments and an abdomen composed of six 

 segments, as well as a caudal plate (telson) ; but the body is stouter 

 and adapted to a more perfect locomotion and a higher grade of 

 life. The thorax, instead of being composed of seven distinctly 

 separate segments, is covered by a dorsal carapace which effects a 

 firm and intimate fusion between the head and thorax. The degrees 

 of development of this dorsal carapace are various. When most 

 highly developed, it forms the dorsal integument of the anterior or 

 of almost all the thoracic segments; and its lateral portions only, 

 which have the form of wings and are bent towards the ventral 

 surface, consist of a free reduplicature. 



The application of the appendages differs from that in the 

 Arthrostraca, and, indeed, varies in the different groups of the 

 Thoracostraca. The cephalothorax has thirteen pairs, and the 

 abdomen seven. The facetted eyes are born on two movably separated 

 stalks. These were for a long time considered as the anterior pair 

 of appendages, while in fact they are merely lateral portions of the 

 head which have become jointed. Both pairs of antennae belong to 

 the anterior region of the head. The anterior antennae or antennules 

 as a rule bear on a common shaft two or three flagella as the 

 peripheral multiarticulate filaments are called and are pre-eminently 

 sense organs. In the Decapoda the auditory vesicles are placed in 

 the basal joint, and on one of the flagella there are delicate hairs and 

 fibres, which are in connection with nerves and are to be looked on 

 as olfactory organs. The second antennae are attached externally to 

 and somewhat beneath the antennules. They bear a long flagellum 

 and in the macrurous Decapoda are often provided with a more or 

 less considerable scale. A gland (the green or antenna! gland) 

 usually opens on a conical process of their basal joint. 



The following three pairs of appendages function as jaws; the 

 powerful mandibles, which are furnished with palps, lie at the side 

 of the upper lip ; further backwards are the two pairs of lobed 

 maxillae, in front of which and behind the mouth is the small bilobed 

 underlip. The following eight pairs of appendages present a very 



" Malacostraca podophthalma Britanniae," London, 1817 1821. V. Thompson, 

 " On the metamorphosis of Decapodous Crustacea." Zool. Journ., vol. ii., 1831, 

 also 7m, 1834, 1836, 1838. H. Bathke, " Untersuchungen tiber die Bildung und 

 die Entwickelung des Flusskrebses,'' Leipzig, 1829. Th. Bell, "A history of the 

 British stalk-eyed Crustacea," London, 1853. Lereboullet, ' Eecherches 

 d'embryologie comparee sur le developpement du Brochct, de la Perch e et de 

 PEcrevisse," Paris. 18G2. V. Hensen, " Studien iiber das Gchororgan der 

 Decapoden." Leipzig, 1863. 



