NATURAL HISTORY OF AMERICAN LOBSTER. 



315 



p.mA 



decapod crustacean but comparable to the flagellate spermatozoa of other animals. 

 The crustacean sperm becomes differentiated in three parts, namely, (i) the nucleus or 

 head, (2) mitochondral body (a partly fibrous and partly granular structure) repre- 

 senting the neck or middle piece, and (3) the explosive capsule or modified tail. 



The sperm cell develops processes (fig. 31,1) which in the lobster arise from the neck. 

 They ordinarily appear to be immobile and are distinctive of the decapod sperm. The 

 number of these processes varies from i to 10, but 3 is a common number, which is 

 found in Homarus, Palinurus, and Galathea, as well as in some of the crabs. The 

 number is of physiological importance, since they are used for orienting the sperm upon 

 the egg in fertilization. In the true crabs the processes arise from the head and are 

 therefore nuclear in origin. It may be added that in the prawns {CarididcB) the usual 

 processes are lacking, but the capsule ends in a sharp thread-like tail. In the crayfish 

 {Astacus) and many crabs, as well as in Gehia and Callianassa, the neck and capsule 

 are reduced in size and pressed against the head. 



The processes are supported by a central mitochondral, skeletal fiber, or bundle of 

 fibers. If in the course of development of the spermatid, 

 these strong skeletal fibers project from the cell body with 

 free ends, appearing to draw after them the more fluid 

 constituents of the cell. The skeletal fibers can be demon- 

 strated by plasmolyzing the cell or surrounding it with a 

 solution of higher osmotic pressure. These skeletal fibers 

 are really bundles of fibrils, which have a tendency to spiral 

 winding. 



The capsule (fig. 31,1 and 2) is a double walled cylin- 

 drical body, a median tube running through it from end 

 to end. This tube is formed by a median invagination ex- 

 tending from the hinder end forward to the neck, and is 

 expanded at either extremity into a widechamber. The 

 distal opening is closed by a plug of chitin. A peculiar rod 

 or "Polster" of stainable substance is pressed from the central body into the anterior 

 chamber. In the ripe sperm the outer capsular wall and the axial tube consist of 

 chitin, and may be regarded as continuous, except at the point pierced by the "Polster." 

 This stainable rod is often constricted into a proximal central body in the neck and a 

 distal central body in the capsule {p and deb). 



-md.t 



fiJ..ijit 



in.t 



Fig. 32. — Diagrammatic section of 

 sperm cell in capsular explosion, as 

 seen in plane a. a, figure 31 (3); jd, 

 in. U, folded inner tube. 



FERTIUZATION. 



In the lobster the sperm cells pass a long latent or resting period in the sperm 

 receptacle, and may retain their vitality for from one to two years, and possibly longer. 

 When the eggs are laid, the sperms leave their receptacle, find the eggs, and fertilize them. 

 The spermatazoa are either pressed out by mechanical force, or else they must be 

 aroused to activity by a definite stimulus, probably of a chemical nature. 



