204 



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

 of the adult Clypcastroids. and accordinj^iy leslity the correctness of the 

 sui»f»estion of the classificatory value of the larv;c. 



The Clypeastroid-Iarvae are characterized by the body skeleton forming 

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

 fenestrated plate in tiu- 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. Xo 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 ClypcaslciAnvya known 

 as yet differs rather conspicuously from the other larva% excepting the 

 luhinocyarmis- and the Laydiium-lavya ( — 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 infornialion is needed before we can form a real judgment of the 

 value of this dilTerence. Another very noticeable fact is this thai 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 larvse of the other Scutellids. 



H is worth j)oinling out thai 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 larvte and the young, meta- 

 morphosing sea-urchin. 



Within the Regular Kchinoids we nuiy first dismiss the order of the 

 Cidaroidea, altogether too little being known of their larva- for judging 

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

 lu-hiiioplutciis Iransix'isiis belongs lo Cidarids, it will be very hard lo recon- 

 ciliale its highly specialized characters with the primitive character of 

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

 larvae as Echinopluleus IransDersus and the larva of Cidaris cidaris (Doro- 

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

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

 blem so long as it has not been definitely settled that Echinoplulms Irans- 

 versus really is a Cidaroid-larva. 



The few facts known about the development of Diadematids lend to 

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



