490 COMPARATIVE ANATOMY CHAP. 



these spherules are enclosed, at least at first, in cells, each of which 

 has a nucleus lying in the base, which is turned away from the 



surface. These are regarded as 

 connective tissue cells. 



According to recent ontogenetic research, 

 each individual spherule is the metamor- 

 phosed product of a mesen chyme cell, and 

 the sacculi on their first appearance are said 

 to be nests of such cells. 



Sacculi are specially numerous in all 



f^^^/^t^^Wv^~^^-^_ species of Antedon, but are also found in 

 \~^&^^^^ > " 4 Eudiocrinm, Promachocrinus, Pentacrinus, 



\/ -.,.,-, ..._,._> Rhizocrinus, Bathycrinus. They are want- 



j, ing in Adinometra, Thaumatocrinus, and 



FIG. 387. -Diagram of a sacculus. 1, Super- Holopus. Their significance has not been 

 licial layer of integument passing over the discovered. They have, been regarded by 

 sacculus ; 2, granular masses within special various authors as calcareous glands, ex- 

 cells ; 5, the nuclei of these cells; 4, nuclei cret of unicellll i ar a i g8e 

 of the surrounding cutis (3). , . 



and slime glands, but, according to the 



most recent opinion, they are proteid corpuscles, deposited in the connective tissue 

 cells as reserve stuff', to be used as occasion requires, for the regeneration of broken - 

 off arms or of the viscera. 



In other Echinoderms the contents of wandering cells (especially of those cells 

 which are massed together below the surface of the Holothurian integument) have 

 also been claimed as reserves of nutrition. 



XIX. Genital Organs. 



A. General Morphology. 



With rare exceptions, which will be dealt with separately, the 

 sexes are separate in Echinoderms. 



The genital organs are throughout distinguished by great sim- 

 plicity, as evidenced by: 



1. The entire absence of every kind of eopulatory organ. 

 The sexual products are ejected from the body, and fertilisation takes 

 place in the water (except in cases of care of the brood to be men- 

 tioned later). 



2. The entire absence of accessory glands, of widenings or 

 outgrowths of the ducts, and of complicated adaptations for the 

 nourishment of the ripening sexual products. 



The genital organs consist of variously shaped tubes, within which 

 the spermatozoa or eggs ripen, and from which they are discharged 

 through simple efferent ducts. 



These gonadial tubes lie in any part of the body cavity ; in the 

 most complicated cases their wall consists, from without inward, of 

 (1) the endothelium of the body cavity; (2) a thin muscle layer; 



