6 



ECHINODERMS OF THE BRITISH ISLES 



OpJiiophiffus (()))hiiir()i(l larvji^), .and Echinopluteus'^ (Echinoid 

 larvae) may all be derived from the same simple original type. 

 The Crinoid larvcC alone eannot be referred to this type ; the 

 few larvae of sea-lilies known at present are all barrel-shaped, with 

 sej)arate ciliated rings and not a single continuous band. Similar 



Fig. 3. — The four main types of Echinoderm larvae. 

 (From Danmark's Fauna.) 



1. Aurindarici, larva of a Holothurian or Sea-cucumber (Synapta). 2. Bijiinnnria (in 

 the Brachiuhtrin stage), larva of an Asteroid or Sea-star (Asterias). 3. Ophioplideus 

 compressus, larva of an Ophiuroid or Brittle-star. 4. Echinopluteus, larva of an Echinoid 

 or Sea-urchin (Echinocardium cordatuni). (1. After Senion ; 2. after A. Agassiz, slightly 

 altered.) A, Anal opening ; ad, Anterodorsal arm ; al, Anterolateral arm ; br, Brachio- 

 larian processes; e, Enterocoel vesicles; h. Rudiment of the watervascular system (the 

 hydrocoel) ; k, Calcareous wheels ; m, Stomach ; n, Nervous system ; o, Mouth ; pd, 

 Posterodorsal ; pi. Posterolateral ; po, Postoral; pr, Preoral arms; r, Rectum; s. Sucking 

 disk ; sk, Skeleton. 



larval forms are known also in the other classes, especially in the 

 Dendrochirote Holothurians, which would all appear to have 

 such barrel-shaped larvae. It j)robably holds good for all echino- 

 derms with large, yolky eggs that they have no true pelagic 



^ The Ophiuroid and Echinoid larva? were originally designated by the 

 common name Pluteus. 



