114 MORPHOLOGY OF INVERTEBRATE TYPES 



in the case of all Cladocera, it is clearly separated from the rest 

 of the body. The somites of which the head in Daphnia pulex 

 is composed are as follows: (i) ocular somite, (2) first antennal 

 somite, (3) second antennal somite, (4) mandibular somite, (5) 

 first maxillary somite, (6) sixth cephalic somite devoid of ap- 

 pendages in the adult but corresponding to the second maxillary 

 somite of all other Crustacea. These six somites are fused to- 

 gether and even in the embryo do not articulate with each other 

 but represent a single unit. The first three somites are preoral, 

 the following postoral in the adult. The remainder of the body 

 is not clearly divided into a definite number of somites and is not 

 well separable into a thorax and abdomen. The five somites 

 following the last cephalic somite are apparent from the five 

 pairs of limbs, whereas the end of the body has no appendages 

 and has lost all evidence of external and internal segmentation. 

 Thus it is impossible to say to how many somites this portion 

 of the body corresponds. The end of it is used as an organ of 

 locomotion when the animal is moving on the bottom of the pool. 

 For this reason it is often called the "scratch foot." It has two 

 tactile bristles, a series of small teeth on the dorsal surface, and 

 ends in two powerful claws which are known under the name of 

 furca. The trunk of the animal is protected by a right and left 

 fold of the integument extending beyond the thoracic append- 

 ages. This fold is often spoken of as the "shell " from its analogy 

 to the bivalve shell of the lamellibranch molluscs. The postero- 

 dorsal end of the shell is drawn out into a sharp angle which cor- 

 responds to the long spine found in other species of Daphnids. 

 The ventral, free edge of the shell carries a series of long bristles 

 at right angles to the surface of the shell and directed inward, 

 i. e., toward the plane of symmetry. These bristles afford an 

 excellent protection to the gills of the animal while at the same 

 time freely admitting water into the shell. 



Integument and skeleton. The integument of Daphnia 

 consists of a single layer of epithelial cells or hypodermis which 

 secretes a thin supporting membrane on its inner surface and a 



