4G 



ECHIN0DERM8 OF THE BRITLSH ISLES 



several more sea-stars, the development is also direct, the larva 

 being small, much reduced, and without a mouth. Most sea-stars, 

 however, have a highly developed larva, Bipinnaria (see p. 6), 

 which leads a pelagic life for a period usually of some weeks, 

 feeding on Diatoms and other small pelagic organisms. In some 

 Bipinnarise (the larvae of Asterias and Porania) the three anterior 



Fig. 28. — Larvae of Asteroids. (From DanmarFs Fauna.) 



1. Yomm Bipinnaria of Luidia (Sarsii/). x 16. (After Garstang.) 2. Biinnnaria 

 of Luidid ,S(irsii, near metamorphosis; in the posterior end is seen tiie young soa-star. 

 X 3. (After Joh. Miiller.) 3. Bipinnaria of Aslropecten irregularis, x 35. 4. Braehio- 

 laria stage of tiie larva of Asterias rubens ; in the posterior end is seen the rudiment of the 

 young sea-star, x 10. br, Bracliiolaria arms ; h, Hydrocoel ; m, Stomach ; oe, (Eso- 

 phagus ; p, Spines ; r, Rectum ; s, Sucking disk ; st, Sea-star. 



ventral arms are provided with small papillae, arranged in a 

 whorl round the point or in a series along each side of the arm. 

 At the base of these arms there is a large sucking disk, by means 

 of which the larva may attach itself (during metamorphosis). 

 Such larva? are said to have a Brachiolaria stage, or are 

 designated by the name Brachiolaria. The arms provided with 

 these papillae are called Brachiolaria arms (Fig. 3, 2 ; Fig. 28). 



