230 ZOOLOGY 
(Fig. 152). 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. 
Fra. 132.— View of blood-vascular system 
of a Starfish as described by German 
writers. Modified from Ludwig. 
1. Cireumoral ring. 
2. Radial 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 Lrisinga 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 ASTROPECTINIDAE is, with one exception, charac- 
terised by the anus being absent, and by the tube-feet being 
