166 ON THE STRUCTURE AND 



The structure of the shell after moulting is frequently altered 

 very considerably at the dorsal portion behind the line of fission, 

 the lozenge-shaped, or polygonal reticulations, as the case may be, 

 being here considerably smaller; indeed, where the ephippium has 

 been cast, the reticulations disappear, and are replaced by irregular 

 puckered markings (see Fig. 21). This is connected, I think, with 

 the growth of the shell, which is more rapid on the dorsal than on 

 the ventral margin, and is requisite to produce the brood-receptacle, 

 for in the young this receptacle scarcely exists, the body occupying 

 the whole cavity. In connexion with this, also, I may mention 

 that Baird describes two varieties of Daphnia pitlex^ one having 

 the spine in a line with the straight dorsal margin of the shell, 

 and the other having it placed in a medial position at its 

 extremity, the dorsal margin shewing as much, or nearly as much, 

 flexure as the ventral. I believe this does not arise from varietal 

 difference of form, but from excessive growth of the dorsal portion 

 of the shell after the production of successive broods of young, 

 for I do not find it in the young animals. 



The mouth of the Daphnia is not easily discovered. It is 

 situated immediately under the labrum or upper lip, and between 

 the grinding surfaces of the mandibles (?;//, Fig. 11). Hither are 

 collected all the nutritious particles that come within reach of the 

 current created by the movements of the feet. This current may 

 be seen to set in between the anterior margins of the valves, as 

 indicated by the arrow in Fig. 2, and the particles are collected in 

 a sort of gutter, commencing with the posterior pair of feet, and 

 extending thence forward between the bases of the limbs to 

 the mouth, where they frequently form a dark mass (//, Fig. 4). 

 The alimentary canal commences with a narrow oesophagus {pes.^ 

 Fig. 11), which passes upward into the head between the crura of 

 the brain; it is furnished with muscles (;;?', Fig. 11) attached to 

 the integument, which occasionally enlarge its diameter so as to 

 allow a pehet of food to pass, and closes again immediately behind 

 it. It corresponds to the fore-gut of the higher Crustacea. It 

 then suddenly enlarges into a spacious cavity, which is continued 

 nearly the whole length of the body, and forms the mid-gut. This 

 cavity combines the functions of stomach and intestine, and is 

 furnished with an outer muscular tunic of circular, and, probably, 

 longitudinal fibres, within which is a glandular epithelium, and 

 within this again a fine soft membranous lining ; under ordinary 

 circumstances indistinguishable from the epithelial coat with which 

 it is closely connected. Sometimes, however, it is separated from 

 the latter by a wide interval, and consequently becomes conspicu- 

 ous (see Fig. 26, m I). I am inclined to think that this happens 



