ENTOMOSTRACA. 



[ 286 ] 



ENTOMOSTRACA 



Char. Free ; freshwater or marine ; body 

 more or less distinctly jointed, mostly con- 

 tained in a horny, leathery, or brittle shell 

 or carapace, formed of one or more pieces, 

 often bivalve ; branchiae attached eitner to 

 the jaws or legs ; legs jointed, and more or 

 less ciliated ; development accompanied by 

 a regular moulting or change of shell, some- 

 times amounting to metamorphosis. 



Many of the Entomostraca are very com- 

 mon in ponds, pools, and other collections 

 of water. When examined with the naked 

 eye, in a bottle or glass containing the water, 

 they appear as minute specks, generally in 

 active and often in jerking motion. 



The shell is often beautifully transparent, 

 sometimes spotted with pigment, variously 

 striated, reticular, or notched, sometimes 

 spinous or tuberculated. It consists of chi- 

 tine impregnated with a variable amount 

 of carbonate of lime, which is sometimes so 

 great as to render it brittle, and to cause 

 copious effervescence on the addition of a 

 dilute acid ; and when boiled it turns red, 

 like the shell of a lobster. It varies in 

 structure, sometimes consisting of two 

 valves, united at the back, resembling the 

 bivalve shell of a mussel ; at others it is 

 simply folded at the back so as to appear 

 bivalve, without really being so ; or it con- 

 sists of a number of rings or segments. It 

 often presents a reticular appearance re- 

 sembling that of a cell-structure. 



The body itself, which is more or less in- 

 timately connected with the shell, is mostly 

 divided into numerous segments. The head 

 is furnished with one or two pairs of an- 

 tennae ; the superior or anterior are usuaDy 

 smallest, and in some genera easily over- 

 looked (PI. 20. fig. 28 a) ; sometimes one or 

 both of them are furnished in the male 

 with a hinge-joint, allowing considerable 

 flexure, so as to permit of its grasping the 

 female (PI. 20. fig. 8 , of male; 9 a of fe- 

 nale) ; sometimes they are long, and pro- 

 vided with a tuft of filaments (PI. 20. figs. 

 17,18); at others they are simply long, and 

 filiform or setaceous (fig. 38). In some 

 Cypridinidae and Concnceciidae, the upper 

 antennae become organs of special sense 

 (smell or hearing), being clothed with 

 toothed club-shaped appendages, which 

 arise directly from the antennae (Conchcecid), 

 or from its "bristle-shaped appendages (Cy- 

 pridina). The inferior pair or posterior 

 antennae vary in size and structure like the 

 former, being sometimes large and branched 

 (fig. 28 6), and serving to row the animals 



through the water, at others resembling 

 legs (tigs. 5, 17, &c.). In some genera they 

 are furnished with curious appendages, 

 effecting the purpose of the hinge-jointed 

 superior antennae. In some, again (Cythe- 

 ridae), the lower antennas (PI. 19. fig. 37) 

 are armed with alougcurved2- or3-jointed 

 urticating seta (6), connected at its base 

 by a duct with a vesicular gland situated 

 in the anterior part of the body (a). An 

 external stalked vesicle is also sometimes 

 found attached to this antenna. 



The eyes are usually large, the pigment 

 black or red, and the muscles and the ner- 

 vous branches distributed to them from the 

 cephalic ganglion very distinct. 



A labrum or upper lip is often present, 

 compressed and terminated by a hairy lobe 

 (PI. 20. fig. 35) ; sometimes also a labium. 

 Behind these are situated two mandibles, 

 furnished with either blunt or pointed teeth, 

 often having a palpus or palp-like organ 

 (fips. 11, 20, 34). Next to these, comes a 

 pair of maxillae, jaws or foot-jaws (figs 12, 

 36), furnished with spines, hooks, or claws, 

 and sometimes branchiae (fig. 21). Behind 

 these is a second pair of foot-jaws (figs. 13, 

 22). The legs are variable in number and 

 structure; they are often furnished with 

 flattened processes, fringed with beautifully 

 ciliated or plumose filaments (figs. 30, 31, 

 32), thus exposing a large extent of surface 

 to the water, by which respiration is effec- 

 ted; hence they represent gills, and are 

 called branchiae or branchial legs or feet ; 

 similar branchiae are often appended to the 

 foot-jaws ; and they are in constant motion, 

 even when the animal is at rest. 



In some genera, as Canthocamptus, Cy- 

 clops, and DiaptomuSy the respiration is ex- 

 clusively anal ; the water being drawn into 

 and expelled from the rectum at intervals, 

 as in many aquatic larvae of insects. 



As the structure and arrangement of these 

 parts afford characters for distinguishing 

 the genera &c., the details are given under 

 their respective names. 



The abdomen is of variable length, join- 

 ted with a variously lobed post-abdomen, 

 often resembling a tail in appearance (figs. 

 3, 8) ; sometimes it is bilobed ; sometimes 

 Burnished with a kind of spur near the end, 

 :or supporting the ova within the shell, 

 tn some genera the external ovaries or ovi- 

 sacs containing the ova pass out between 

 two of the abdominal joints, yet remaining 

 attached, and giving a remarkable appear- 

 ance to the animals (figs. 9, 38). The in- 



