106 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



its wide mouth the Uastopore (bp), which soon narrows, and even- 

 tually closes. Owing to the presence of food-yolk, which the 

 ectoderm envelops, this layer is much more extensive than the 

 endoderm. The mesoderm (mesoblast) is recognizable at an early 

 stage as a solid mass of cells arising in front of the archentercm, 

 and derived from its wall. It gradually extends, by division of 

 its elements, until it forms a complete layer surrounding the body 

 within the ectoderm. 



2. General Growth. The germinal disc is the first trace of the 

 ventral surface of the embryo. From its hind end the abdomen 

 grows out as a small knob, and on its front end and sides little 

 elevations appear, the rudiments of the eyes, labrum, and append- 

 ages of the head and thorax. The swollen part containing food- 

 yolk corresponds to the dorsal region of the cephalothorax, and 

 it gradually becomes smaller as its contents are used up in the 

 processes of growth. 



The young crayfish are hatched at or near the commencement 

 of summer. "They resemble the adult in most respects, but the 

 first and last abdominal appendages are wanting, and the ends 

 of the forceps are sharply hooked, thus serving as a means of 

 attachment to the cement with which the swimmerets are more 

 or less covered. 



Owing to the firm exoskeleton, growth cannot take place in 

 the same way as in soft-bodied animals, and the adult size is only 

 reached after a series of moults or ecdyses, by which the cuticle 

 (including the endophragmal system and the linings of the fore- 

 and hind-guts) is thrown off, a fresh one being subsequently 

 secreted. Moulting occurs as often as eight times the first year, 

 five the second, and after that less frequently. 



3. Fate of the Germinal Layers. (1) The ectoderm (epiblast) 

 develops into the epidermis, with the exoskeleton and its various 

 types of seta?. The epithelium and cuticle of the fore- and hind- 

 guts are also derived from this layer. They both arise as deep 

 pits which become connected with the archenteron to form a 

 continuous alimentary canal. The nervous system is formed from 

 ectodermal thickenings, and the auditory sacs from ectodermal 

 pits. Each eye (except the pigment, which is mesodermal) is 

 developed from (1) an ectodermal thickening, and (2) an optic 

 pit or fold of ectoderm internal to this. All external to the 

 retinulse arises from (1), while (2) becomes isolated from the 



