POD OPH THA LMIA . 



45 



The thoracic feet in some of the lower groups retain, even when adult, the exopo- 

 dite, though in a very rudimentary condition. In the higher forms it always entirely 

 disappears. One or more of these thoracic feet usually terminate in a pincer or chela, 

 which can be used in grasping or crushing. In the lobster these pincers acquire a 

 great development, and no mention is necessary of the strength with which they can 

 pinch. At first sight it would appear as if, in order to form the pincer, the end of the 

 leg was split, but a little examination shows that in reality the chela is produced by 

 the last joint of the leg meeting a prolonged portion of the preceding joint, and in 

 some forms (e. g. Crangori) intermediate stages can be seen where this inner angle is 

 not produced so far, and the terminal joint has to bend at right angles in order to 

 meet the palm. 



Among the abdominal feet the amount of variation is but slight, and when present, 

 all, except the first pair, which are modified for reproductive purposes, are biramous ; 

 the last pair in the Macrura uniting with the last segment of the body, the telson, to 

 form the powerful caudal fin which the animal uses in swimming backward through 

 the water. As will be seen when we come to the subdivisions of the Decapoda, the 

 abdomen shows a good deal of variation in size and development, which need not here 

 be discussed. 



We have already referred to the position of the mouth, which is situated between 

 the mandibles. In front w r e have a fleshy upper-lip, and below an under-lip, which in 

 every way, both in structure and development, appears to be an appendage homologous 

 with the others, but which, until the present decade, has never been so considered. 

 Respiration is effected (with a very few exceptions) by gills, which are always present, 

 and are attached to the basal joints of the thoracic limbs and a few of the mouth- 

 parts, and extend up into the gill-chamber already described. The heart is a strong, 

 small, and compact organ, situated in the middle of the upper part of the thorax. 

 From it arise five arteries ; one supplying the eyes, one the upper surface of the 

 abdomen, one goes down to the lower surface of the body, and there dividing, supplies 

 the appendages, while the two remaining arteries, one on each side, convey the blood 

 from the heart to the viscera. 



The eggs of the Decapods, after exclusion from the genital organs, pass back, and 

 are attached to the abdominal feet of the mother, where they undergo a portion of 

 their development. The eggs of the Decapods have a regular segmentation, but, owing 

 to the fact that the protoplasm has a perpheral distribution, the planes of cleavage do 

 not pass completely through the egg. When the segmentation is completed, a small 

 patch of the blastoderm invaginates to form the primitive stomach, and the opening or 

 blastopore soon closes. By similar imaginations the anterior and posterior portions of 

 the digestive tract are formed, each pushing in until it meets the first invagination. 



C3 / J. ^j j 



Anticipating for a moment our account, we will say that here, as in all Crustacea, the 

 primitive stomach, or mesenteron of embryologists, foi-ms but a very small portion of 

 the alimentary tract, and that from the anterior invagination, the stomodeum, as it is 

 called, the stomach with its complicated mill is developed. Hence it is that when the 

 animal casts its skin, the lining of the stomach is also shed. Returning now to our 

 egg, the next feature seen is a small prominence, the future abdomen, and then the 

 first three pairs of appendages appear nearly simultaneously, giving rise to the 

 nauplius stage. Lucifer (which presents an exception to the other Crustacea, in hav- 

 ing a total segmentation and in the formation of a true segmentation cavity) and Peneus 

 leave the egg at this point, and we will follow their further development at another 



