4i6 PHYSIOLOGY OF THE INVERTEBRATA. 



medium tlie embryos are developed ; but in the case of 

 Opltiolepis ciliata, the embryo is developed within the body 

 cavity of the parent. The early stages of the embryological 

 development of most Ophiitridca are similar to those of other 

 Echinodcrmata ; nevertheless, in some forms the embryo 

 passes directly into the adult condition without first becom- 

 ing an Eclmiojjoidmm. " Where an Ecltinopaxlium stage 

 exists, the larva is a Plutcus. The dorsal wall of the 

 body of the embryo exhibits a medium conical outgrowth ; 

 along the course of the ciliated band symmetrically disposed 

 processes are developed ; and these outgrowths are supported 

 by a calcareous skeleton, which is also bilaterally sym- 

 metrical." 



In the Echinidea* there are five unpaired ovaries or testes, 

 which are situated inter-radially. As a rule, they project far 

 into the body cavity, and are composed of ramified tubules. 

 The ducts of the genital glands open externally by five 

 apertures in the genital plates which surround the apical 

 pole. The early stages in the embryological development of 

 Echinus are similar to those of the starfish. The free- 

 swimming larva, however, assumes the Plutcus form, and in 

 this respect it is similar to the Ophiuridca. The Plutcus 

 form has eight long slender arms, which are supported by 

 calcareous rods extending from the body. The adult form 

 of Ecliinus is developed within the body of the larva, the 

 alimentary canal and dorsal sac (the commencement of 

 the ambulacral system) alone persisting. The larval body is 

 gradually absorbed by the developing and growing Echinus, 

 the spines and pedicels of the latter increasing in size and 

 number until the animal assumes the adult form. 



It may be remarked that Professor A. Giard t has shown 

 that at certain times the genital glands of the Echinidea 

 secrete small crystalline concretions of a brownish colour. 

 These concretions consist of calcium phosphate, which is 



* The sexes are distinct. 



t Comptes Rendus de V Academic des Sciences, 1877. 



