266 ECHIN0DERM8 OF THE BRITISH ISLES 



fi. Body skeleton forms a basket structure^ 



( Echinocyamus pusillaa 

 \ I. stage of Sphcerechinus granular is 

 Body skeleton forms no basket structure .... 7 



7. Postoral and postero - dorsal rods fenestrated, or, at least, 



composed of 3 parallel rods ...... 8 



Rods all simple ........ 9 



8. Postoral and postero -dorsal rods imfenestrated, or, at most, 



fenestrated in the outer part, but composed of 3 parallel 

 rods . . . . . .1. stage of Arbacia lixula 



Postoral and postero-dorsal rods fenestrated throughout their 

 whole length (Fig. 183,2) . Older larva, oi Echinocyamus pusillus 



9. Posterior end of body elongate, supported by the long, more or 



less club-shaped body rods (see below) 



I. stage oi Psammechinus, Echinus, 

 Paracentrotus, and Strongylocentrotus 



Posterior end of body rounded, not elongate ; body rods short, 

 simple. Epaulettes present . . . . .10 



10. Four epaulettes at the base of the postoral and postero-dorsal 



arms, no epaulettes at posterior end of body (see below) 



II. stage of Psammechinus miliaris 



and Paracentrotus lividus 

 Besides the four epaulettes at the base of postoral and postero- 

 dorsal arms also a pair of epaulettes (or a ciliated ring) at the 

 posterior end of the body (see below) 



II. stage of Echinus esculentus, Ech. acutus, 

 and Strongylocentrotus drobachiensis 



11. Postero -lateral arms long, with skeletal rod . . .12 

 Postero - lateral arms short, ear-shaped, without skeletal rod 



(Fig, 201) ..... . Brissopsis lyrifera 



12. Postero -lateral rods widened at the base, the widened part 



formed by three thin lists, at an angle to each other (Joh. 



Miiller, VII. Abhandl. Taf. VII.) ' Echinopluteus fusus Mrtsn. 



Postero-lateral rods simple . . . . . .13 



^ It is hardly possible to indicate how the young stage of Sphcerechinus 

 granularis and Echinocyamus pusillus can be distinguished : as soon as the 

 postero-dorsal rod has appeared, they are easilj^ distinguished, this rod 

 being fenestrated in Echinocyamus, simple in Sphcerechinus ; further, 

 a posterior transverse rod is never formed in Echinocyamus. No doubt 

 also the (unknown) young larva of Centrostephanus longispinus will prove 

 to have a basket structure. 



^ It is unknown to which Spatangoid this larva belongs ; it is known 

 both from the Mediterranean and the Atlantic, and may well be expected 

 to occur also in British seas. Also in the Indo -Pacific Ocean Spatangoid 

 larvae with this characteristic type of postero-lateral rods occur, so that it 

 must be assumed that several species of Spatangoids have a larva of this type. 



