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, 

 eacli individual spherule is the metamor- 

 phosed product of a mesenchyme cell, and 

 the sacculi on their first appearance are said 

 to be nests of such cells. 



Sacculi are specially numerous in all 

 species of Antedon, but are also found in 

 EinUocriims, Promachocrinvs, Penlacrinns, 

 Ehhocrinns, Batliycrinus. They are want- 

 ing in Actinometra, ThawncUocrimis, and 

 Holopus. Their significance has not been 

 discovered. They have been regarded by 

 various authors as calcareous glands, ex- 

 cretory organs, groups of unicellular algse, 

 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 Echinoder)ns 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. 



Fig. 3S7.~Diagramof asacculus. 1, Suppi- 

 licial layer of inte^'umeut passing over the 

 sacculus ; 2, granular masses within special 

 cells ; 5, the nuclei of these cells ; 4, nuclei 

 of the surrounding cutis (3). 



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; 



