CILETOPODA 309 



into the sea-water automatically. One great cause of the 

 absence of special habits or instincts, is the rudimentary state 

 of the nervous system. There are distinct strands of nervous 

 tissue, but no differentiated ganglia, the muscular system and 

 sensory organs are very slightly developed, and sexual instincts, 

 for the development of which sensory organs and developed 

 nervous centres are necessarily required, accordingly do not 

 exist. 



There are cases, however, among the Echinodermata in 

 which the eggs are kept during development in some kind of 

 external brood-pouch, and this structure, when confined to 

 the females, constitutes a secondary difference between the 

 sexes. The common brittle-star Amphiura squamata is vivi- 

 parous, but here the brood-pouch is a portion of the genital 

 apparatus which occurs in both sexes. In some Holothurians, 

 however (Psolus ephippiger), there is a brood-pouch on the 

 back of the females only, a condition similar to that 

 which occurs in certain Amphibia. The receptacle is formed 

 by calcareous plates . supported by rods projecting from the 

 skin. In this case the special structure is a modification of 

 parts originally existing, and unmodified in the male. The 

 modification is evidently dependent on the stimulation pro- 

 duced by the presence of the eggs and embryos, without which 

 stimulation the "variations" required to give rise to the 

 modification would not have occurred. 



Coelenterata. In this sub- kingdom, including jelly- 

 fishes, polyps, corals, and sea-anemones, secondary sexual differ- 

 ences are generally absent, and for the same reasons as in the 

 Echinodermata. Here also, however, occasional cases occur 

 in which some slight difference in the female are developed 

 in association with protection of the eggs or larvae. In the 

 commonest jelly-fish of our coasts, Aurelia aurita, the female 

 differs slightly from the male. The difference consists in the 

 form and length of the arms or tentacles surrounding the 



