230 



ZOOLOGY 



(Fig. 132). The outer cells of this ridge are mainly sense 

 cells, and ganglion cells and nerve fibres occur at their bases. 

 The nerve layer is also well developed on the tube-feet. 



Fig. 132. — View of blood- vascular system 

 of a Starfish as described by German 

 writers. Modified from Ludwig. 



1. Circumoral ring. 



2. Eadlal vessel with branches to am- 



pulla. 



3. Heart. 



4. Circumoral ring. 



5. Dorsal end of heart passing into the 



skin. 



6. Vessels to intestine. 



7. Paired vessels passing to generative 



glands. 



Asterias rubens is dioecious. The generative organs con- 

 sist of five interradial pairs of glands, which are alike in both 

 sexes, and when mature each extends into two neighbouring 

 arms (Fig. 133). Except during the breeding season, the size 

 of the glands is inconsiderable. Each gland opens to the 

 exterior by a single duct, which terminates in a perforated 

 plate situated dorsally and interradially. The various glands 

 are connected together by a genital rhachis, and they are 

 supplied by the above-mentioned genital vessels, which dilate 

 to form a sinus round the glands. Fertilisation takes place 

 externally. 



The Asteroidea are mainly inhabitants of shallow water, 

 though a considerable number of species from great depths have 

 been described. The arms are usually five in number; one 

 species of Solaster has, however, thirteen, and Brisinga has nine 

 to twelve arms, which are more sharply marked off from the disk 

 than is the case with other Asteroidea. The same genus is 

 devoid of dermal branchiae, of eye-spots, and of ampullae at the 

 base of the tube-feet. 



The family Asteopectinidae is, with one exception, charac- 

 terised by the anus being absent, and by the tube-feet being 



