ENTOMOSTRACA. 



[ 2S0 ] 



ENTOMOSTRACA. 



mon in ponds, pools, and other collections 

 of water; when examined with the naked 

 ej'e, in a bottle or glass containing the water, 

 they appear as minute specks, generally in 

 active and often jerking motion. 



The shell is often beautifully transparent, 

 sometimes spotted with pigment ; it consists 

 of chitine 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, 

 just 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 consists of a 

 number of rings or segments. It often pre- 

 sents a reticular appearance resembling that 

 of a cell-structure. 



The body itself, which is more or less 

 intimately connected with the shell, is mostly 

 divided into numerous segments. The head 

 is furnished with one or two pairs of antennae; 

 the superior, — sometimes called also the 

 anterior, the antennae simply, or the anten- 

 nules, — are usually smallest, and in some 

 genera easily overlooked (PI. 15. fig. 28 a) ; 

 sometimes one or both of them are furnished 

 in the male with a hinge-joint, allowing con- 

 siderable flexure, so as to permit of its 

 grasping the female (PI. 15. fig. 8 a, of male ; 

 9 a, of female) ; sometimes they are long, 

 and provided with a tuft of filaments 

 (PI. 15. figs. 17, 18); at others, they are 

 simply long, and filiform or setaceous (fig. 

 38). The inferior pair, — called also anten- 

 nules, posterior antennae, antennae simply, 

 rami, or second pair of antennae, — vary in size 

 and structure like the former ; being some- 

 times large and branched (fig. 28 b), and 

 serving to row the animals through the 

 water, at others resembling legs (fig. 5, 17, 

 &c.). In some genera they are furnished 

 with curiovis appendages, effecting the pur- 

 pose of the hinge-jointed superior antennae. 



The eyes are usually large, the pigment 

 black or red, and the muscles and the nervous 

 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 

 (fig. 35). Behind this are situated two 

 mandibles, furnished with either blunt or 

 pointed teeth, often having a palpus or palp- 

 like organ (figs. 11, 20, 34). Next to these, 

 come a pair of maxillae, jaws, or foot-jaws 



(figs. 12, 36), fmnished with spines, hooks 

 or claws, and sometimes branchiae (fig. 21). 

 Behind these are a second pair of foot-jaws 

 (figs. 13, 22). The legs are variable in 

 number and structure ; they are often fur- 

 nished 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 effected, hence they represent 

 gills, and are called branchiae or branchial feet; 

 similar branchiae are often appended to the 

 foot-jaws ; and they are in constant motion, 

 even when the animal is at rest. 



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, jointed, 

 and often resembles a tail in appearance 

 (figs. 3,8); sometimes it is bilobed; sometimes 

 furnished with a kind of spur near the end, for 

 suppqrting the ova within the shell. In some 

 genera, the external ovaries containing the ova 

 pass out between two of the abdominal joints, 

 yet remaining attached, and giving a remark- 

 able appearance to the animals (figs. 9, 38). 

 The intestinal canal is usually straight or but 

 slightly curved, sometimes, however, it is 

 coiled (fig. 7) ; the Entomostraca are mostly 

 herbivorous, although some are carnivorous. 

 The sexes have not been distinguished in all 

 the Entomostraca, although in some they are 

 perfectly distinct. It appears also that in 

 certain of them, reproduction takes place 

 according to the law of alternation of gene- 

 rations — females only being producedthrough 

 several generations, and the males appearing 

 only at certain seasons. 



The spermatozoa are often of most remark- 

 able structure (see Spermatozoa). The 

 ova are mostly rounded; sometimes they are 

 covered with spines, and often brilliantly 

 coloured. They are either hatched in the 

 external ovaries mentioned above, or in a 

 space between the body of the parent and 

 the posterior part of the shell, or they are 

 deposited in masses upon and glued to water- 

 plants, and hatched independently of the 

 parents. 



At particular seasons of the year, the ova 

 in certain species are furnished with thick 

 capsules, and imbedded in a dark opake sub- 

 stance, presenting a minutely cellular appear- 

 ance, and occupying the above-mentioned 

 interspace between the body of the animal and 

 the back of the shell (fig. 37 a). This is called 

 the ephippium, and the ova — ephippial or 



