ECHINODERMATA OF NEW ENGLAND. 57 



Primnoa. Central axis horny, branched ; cortical layer 

 hard, with difficulty separated from thesclerobase. 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 the^Echinodermata, "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 



