260 PHYLUM ECHINODERMA. 
forming the carbonate of lime skeleton, but facts are 
wanting. , 
The sexes are separate, and they are like one another, 
both externally and internally. The gonads develop periodi- 
cally, and lie in pairs in each arm. Each is branched like 
an elongated bunch of grapes, and is surrounded by a 
“blood sinus.” Each has a separate duct, which opens 
on a porous plate, between the bases of the arms on the 
dorsal surface. In Asterina gibbosa, however, the eggs are 
extruded ventrally. In the same species there is an in- 
teresting sexual variability: many are first males and then 
females (protandric), others are simply hermaphrodites, 
others seem exclusively of one sex. The eggs of starfishes 
are fertilised in the water, and the free-swimming larva is 
known as a Bipinnaria or as a Brachiolaria. 
Other Starfishes 
Parental care is incipient among Asteroids. A species of 
Asterias has been seen sheltering its young within its arms: 
there is a definite brood-pouch in the form of a sort of tent 
on the dorsal surface of Preraster. 
Many Asteroids break very readily, or throw off their 
arms when these are seized. The lost parts are slowly 
regenerated, and strange forms are often found in process 
of regrowth. Thus the “comet form” of starfish occurs 
when a separated arm proceeds to grow the other four. 
There are many deep-sea forms, such as the ophiuroid- 
like Brisinga, the widely-distributed Aymenaster, and the 
blue Porcellenaster ceruleus; but the majority occur in 
water of no great depth. 
Asteroidea first occur in Silurian strata. 
Classification.— 
Order I. Phanerozonia. With strongly developed marginal 
plates, the upper and lower marginals in contact ; with skin- 
gills restricted to the dorsal (abactinal) surface; with broad 
ambulacral plates; with prominent adambulacrals in the peri- 
stome, with pedicellariz sessile (if present), with two rows of 
tube-feet. ; 
e.g. Astropecten, Luidia, Porania, Asterina, Palmipes. 
Order II. Cryptozonia, - With indistinct or rudimentary marginal 
plates in the adults, often with intermediate plates between the 
