160 ANIMAL KINGDOM. 



fig. 157 the inter-ambulacral areas are broad and the plates large, 

 but the ambulacral are narrow and the plates indistinct. 



The ?»o!^7i-opening is situated below at the centre of radiation of the plates. 



The anal opening in the Regular Echinoids (fig. 153) is at the opposite or 

 dorsal centre of radiation. Around the anal opening there are five minute ova- 

 rian openings. 



In the Irregular Echinoids — constituting a large group — the anal opening is 

 to one side of this dorsal centre of radiation, and often on the ventral or under 

 surface of the animal. In fig. 157, for example, the anal opening is marginal 

 instead of central, while the ovarian pores are at the dorsal centre, as in the 

 Regular Echinoids. 



To one side of the dorsal centre (to the right of the front side), in the Regu- 

 lar Echinoids, there is a small porous prominence on the shell, often called the 

 madreporic body, from a degree of resemblance in structure to coral. In the 

 Irregular Echinoids this madreporic body is in the centre of dorsal radiation. 



The ambulacral areas are sometimes perforated through their whole length. 

 But in other cases only a dorsal portion is perforated, as in fig. 157, and, as this 

 portion has in this case some resemblance to the petals of a flower, the ambu- 

 lacra are then said to be petaloid. A large part of the Echinoids have a circle of 

 five strong, calcareous jaws in the mouth; in another portion there are no jaws. 



The Echinoids have been divided into — 



I. Regular Echinoids. 



1. Cidaris Family. — Having the inter-ambulacral spaces consisting of two 

 series, — the general fact in Echinoids, as above stated. 



2. The Archseocidaris Family, having the inter-ambulacral spaces consisting 

 of more than two series of plates — a peculiarity confined to the Palaeozoic 

 Echinoids. 



II. Irregular Echinoids. 



1. Galerites Family. — Mouth-opening central, furnished with jaws; ambula- 

 cral area perforate throughout. (Found only fossil.) 



2. Clypeaster Family. — Like the preceding, but ambulacra petaloid. (Fossil 

 and recent.) 



3. Eclunoneus Family. — Like the Galerites family, but no jaws. (Recent.) 



4. Cassidxdus Family. — Like the Clypeaster family, but no jaws. (Fossil 

 and recent.) 



5. Spatangus Family. — Mouth-opening not central, and having a bilabial 

 form instead of round or stellate; ambulacra petaloid; no jaws. (Fossil and 

 recent.) 



6. By 'saster Family. — Ambulacral areas not radiating from a common area on 

 the back, but two dorsally distant from the others. Whether there were jaws or 

 not is undetermined. (Fossil.) 



B. Asterioids. — The Asterioids (Star-fishes) have the mouth below ; 

 and there are ambulacral pores and sucker-feelers along the middle 

 of the under surface of the rays. The groups are,— 



1. Asterias Family {Asteridce) .— Eays broad; viscera extending into 

 the rays. 



