and its supposed Parasite. 221 



Synapta, is the next question with which we have to deal. It 

 cannot take place by the moUuskigerous sac remaining passive, 

 as in the cephalic attachment. The attachment to the intestinal 

 vessel must, on the contrary, be an act in the life of the moUuski- 

 gerous sac, an act, however, which is still entirely withdrawn 

 from our inspection. 



With regard to the development and early vital history of the 

 moUuskigerous sac, its occurrence in the full-grown Synapta 

 furnishes no other information than is to be found in the obser- 

 vations of J. Miiller. In the full-grown Synapta, the moUuski- 

 gerous sac is never found, at any period of the year, otherwise 

 than in the sexually mature and moUuskigerous state, and at- 

 tached in the manner above described. 



III. The young forms 0/ Synapta digitata, Milll, 



Quatrefages, in his observations upon Synapta Duverncea (An- 

 nales des Sciences Nat. 2 ser. 1842, p. 73), has pointed out 

 that, much as these Holothurise might be observed on the shores 

 of the Mediterranean and in the Atlantic, they are yet never 

 met with otherwise than in a perfectly developed and sexually 

 mature state, and not less than 6 inches in length. This ap- 

 plies also to Synapta digitata. The non-appearance of smaller 

 specimens is explained by Quatrefages upon the well-founded sup- 

 position that these vermiform Echinoderms, in passing through 

 a metamorphosis, will have a larval form, which, no doubt, will 

 be quite different from that which they afterwards assume. 



Among the numerous forms of swimming larvae of Echino- 

 derms discovered by J. Miiller, are some whose metamorphosis 

 could be traced into animals which, from their general anatomi- 

 cal characters, are Holothurise. These are the Auricularice re- 

 markable for their singular form and compared to a rococo 

 coat-of-arms (Abhandl. der Akad. der Wiss. 1848, p. 98; 1849, 

 p. 35; and 1850, p. 37). Of these larvae J. Miiller has made 

 known two different and apparently widely distributed forms ; 

 both were first observed at Marseilles in spring, then at Nice in 

 August and September, and lastly at Trieste, one only in the 

 spring, and the other late in the summer. Their external distinc- 

 tions are, that one bears in the middle of the posterior extremity 

 of the body a calcareous gland running out into radiate teeth — in 

 fact, a calcareous star, and also, in the lobes into which the body 

 of the larva is prolonged all round, a garniture of clear, pale-red 

 spherules, not composed of lime ; the other larva is destitute of 

 these spherules, and has in the two lobes of the posterior end of 

 the body which contains the anus, besides an inconstant cal- 

 careous sphere, a variable number of very regular calcareous 

 wheels. The young Holothurise (still swimming by means of bands 



