ECHINODERMATA OF NEW ENGLAND. 57 



Primnoa. Central axis horny, branched ; cortical layer 

 hard, with difficulty separated from the sclerobase. Caly- 

 cles protuberant, with scales. Calicular apertures (mouths) 

 with eight scales. Cortical layer rough on external surface. 



* P. reseda. 



IV. ECHINODERMATA . 



The animals which are included in the Echinodermata 

 are all marine and are distinguished by a spinous integu- 

 mentary covering. The integument may be filled with 

 calcareous deposit in the form of sharp, pointed, immov- 

 able warts, or plates closely joined together, bearing sculpt- 

 ured and fluted movable spines. In some cases the 

 integument is smooth and has embedded anchor-shaped cal- 

 careous spicules. The existence of spines has given the 

 name of theJGchinodermata, "hedge-hog skinned," to the 

 group. 



The form of the body varies very considerably. It has 

 sometimes the form of an oblate sphere with immovable 

 calcareous plates, as in the sea-urchins. In others the 

 body is soft and vermiform. The majority are star-shaped, 

 in which a central body and peripheral rays can be differ- 

 entiated. In the ordinary star-fishes the body and rays are 

 with difficulty distinguished. In the group of brittle-stars 

 the body is sharply marked off from the rays which extend 

 as long, 'highly flexible, worm-like bodies. These rays 

 may be filamentous, as in the feather-stars, or divided and 

 subdivided as in the basket-fishes. In the common star- 

 fish we have ordinarily but five rays, while in the sun- 

 stars the rays are numerous. In the pentagonal star-fishes 

 the interval between the rays is filled up, the tip only 

 extending beyond the five angles, and the distinction be- 

 tween the central body and peripheral arms is almost lost. 



The star-shape disappears wholly in the sea-urchins 



