ECHINODERMATA. 2 1 9 



Thus the Echinus is completed, and becomes either a male by the 

 development of testes, or a female by that of ovaria. The ova are 

 impregnated, and develope the monadiform embryo : this is probably 

 metamorphosed into the beroiform Pluteus, which produces, by 

 " metagenesis," rather than by " metamorphosis," the young Echinus^ 

 which completes the cycle by growing into the oviparous adult.* 



In so defining the nature of this process in the work above cited, 

 I wished to express the fact, subsequently recognised by an able com- 

 mentator on Miiller's original and admirable observations f, viz., that 

 the new or echinoid structure throws off a large part of the pluteal 

 larva in and from which it was developed. 



In the metamorphoses of the Holothurice the nascent mature form 

 unites with the larval one, no part of the latter being thrown off. 

 The larva of the Holothurice is bean-shaped, convex dorsad, concave 

 ventrad. An irregular transverse fissure answers to the hilum of the 

 bean, and in this the mouth is placed. The margins of the fissure 

 are edged by a ciliated fringe exactly similar to that of the Pluteus. 

 The anus opens on the ventral surface of the larva, behind the fringe, 

 which forms a continuous circle, the anterior part of which is bent 

 back to form the anterior margin of the fissure in which the mouth 

 lies. In the course of its growth the margins of the larva and the 

 corresponding parts of the fringe are thrown into numerous lateral 

 processes, which give it a scolloped appearance. A portion of the 

 dorsal integument of the larva is inverted towards the stomach, as a 

 tube terminated by an enlarged globular extremity, whose cavity 

 communicates with the exterior and is ciliated internally. The 

 vesicle, which terminates this tube, now sends forth processes, form- 

 ing a "rosette," which lies close upon the stomach. The rosette 

 becomes the circular canal of the chylaqueous system, from which 

 caeca are given oiF anteriorly to form the tentacles, posteriorly to form 

 the chylaqueous canals. The former mouth of the larva is obliterated, 

 and a new and permanent one is formed in the centre of the circular 

 canal and its tentacular appendages. In the meanwhile the 

 Polian vesicles are developed from the circular canal, and a deposit 

 of calcareous matter takes place round a portion of the tubular canal, 

 from whose globular extremity the chylaqueous system has been 

 formed. That portion of the tubular canal which lies between the 

 dorsal parietes and the calcareous deposit dies away, and the re- 

 mainder hangs freely from the circular canal as the "sand canal." 



The rose-coloured ova of the Sipunculus niidus are found in vast 

 numbers in the breeding season floating in the chylaqueous fluid of 



* XXX. p. 22. t CLXXVII. p. 9. 



