204 



form a distinct larval "order", corresponding to the natural order 

 of the adult Clypeastroids, and accordingly testify the correctness of the 

 suggestion of the classificatory value of the larvae. 



The Clypeastroid-larvae are characterized by the body skeleton forming 

 a basket structure, which very often develops into a large complicate, 

 fenestrated plate in the posterior end of the body. Although the body 

 skeleton is partly absorbed in the fully formed larva, there is no change 

 in the body shape, so that we cannot speak of a first and a second larval 

 stage. No posterior transverse rod is formed, and there are no vibratile 

 epaulets, whereas there are small ventral and dorsal vibratile lobes. The 

 postoral and posterodorsal rods are generally fenestrated. 



The question whether different groups of these larvae, corresponding to 

 the families of the Clypeastroids, are to be distinguished, cannot be an- 

 swered at present. It is noticeable that the single Clypeaster-larva known 

 as yet differs rather conspicuously from the other larvae, excepting the 

 Echinocyamus- and the Laganum-larva ( the modified larva of Peronella 

 Lesueuri does not count in this connection ), in its skeleton being much 

 simpler, apparently not forming a fenestrated plate in the posterior end 

 of the body; this may possibly indicate a family character. But very much 

 more information is needed before we can form a real judgment of the 

 value of this difference. Another very noticeable fact is this that the larva 

 of Arachnoides placenta, so unique among the Clypeastroids through the 

 aboral position of its periproct, does not differ in any way markedly from 

 the larvae of the other Scutellids. 



It is worth pointing out that the peculiar property of turning green on 

 preservation in alcohol or on being damaged or dying, so characteristic 

 of Clypeastroid tissue, is found also in the larvae and the young, meta- 

 morphosing sea-urchin. 



Within the Regular Echinoids we may first dismiss the order of the 

 Cidaroidea, altogether too little being known of their larvae for judging 

 about their essential characters. If the unexpected should turn out that 

 Echinopluteus Iransversus belongs to Cidarids, it will be very hard to recon- 

 ciliate its highly specialized characters with the primitive character of 

 Cidaroid structure, as also it would be most surprising to find so different 

 larvae as Echinopluteus transversus and the larva of Cidaris cidaris (Doro- 

 cidaris papillala) within the same family (the Cidarids apparently forming 

 only a single family). - - But there is no reason to trouble with this pro- 

 blem so long as it has not been definitely settled that Echinopluteus trans- 

 versus really is a Cidaroid-larva. 



The few facts known about the development of Diadematids tend to 

 show that their larvae are characterized by their skeleton forming in their 



