ECHINOIDEA 265 



absorbed and transformed on the passing from the I. to the 

 II. stage. Contrary to the Ophiuroid larvae, in which the skeleton 

 consists of only two symmetrical halves, the larval skeleton of 

 the Echinoplutei consists, in the fully formed larvae, of four paired 

 parts, viz. (1) the body skeleton with the postoral and antero- 

 lateral rods, and (2) the postero-dorsal rods ; further, there is an 

 unpaired rod, the dorsal arch, from which proceed the pre-oral 

 and antero-dorsal rods. In most Echinoid larvae (but not the 

 Echinus larva) still another unpaired skeletal part, the posterior 

 transverse rod, from which issue the rods of the posterolateral 

 arms, and, in Spatangoid larvae, the unpaired posterior rod 

 (Fig. 3, 4). When postero-lateral arms are developed, they are 

 directed straight to the sides or obliquely backwards, never 

 forwards, as is the case in the Ophiuroid larvae. On the body 

 skeleton there is often a '' recurrent " rod, which, together with 

 the body-rod, forms a sort of '" basket ", e.g. the larva of 

 Echinocyamus -pusillus (Fig. 181, 1). Some larval forms have in 

 the II. stage peculiar "ciliated lobes" or "epaulettes". Upon 

 the whole the Echinoid larvae are rather complicated, but very 

 characteristic and very beautiful organisms. 



Key to the Echinoid larvcE {Echinopluteus) known from or likely 

 to occur in British seas. 



1. An unpaired process from the posterior end of the body . 11 

 No „ „ „ „ „ .2 



2. A posterior transverse rod present ..... 3 

 No „ „ „ „ 6 



3. Postero-lateral arms well developed (Fig. 165) . Arhacia lixida 

 Only short postero-lateral lobes, not developed as arms . 4 



4. Four broad epaulettes present ; postero-dorsal rod unfenestrated 



(Fig. 180) . . . . . Sphoer echinus granular is 



No epaulettes ; both postoral and postero-dorsal rods fenestrated 5 



5. Postoral and postero-dorsal arms very long, about three times the 



body length, directed nearly horizontally towards the ventral 

 and dorsal sides. Ciliated lobes very large ( Joh. Miiller, VII. 

 Abhandl. Taf. V. 1-4)1 ^ (t) Centrostephanus longispinus 



Postoral and postero-dorsal arms only about the length of the 

 larval body, not ventral- and dorsalwards directed and not 

 nearly horizontal. Ciliated lobes not so very large. 

 (Fig. 152)- ...... Cidaris cidaris 



1 In the author's Echinodermenlarven d. Plankton Exped., p. 79, this 

 larva is designated as Echinopluteus MuUeri. There can, however, be Httle 

 doubt about its belonging to Centrostephanus longispinus, so that it would 

 seem superfluous to retain the name Echinopluteus MiXlleri. 



2 It would seem probable that the larva figured by Joh. Miiller, 

 VII. Abhandl. iiber Echinodermenlarven, Taf. I. 1-2, is also a Cidarid larva, 

 and if so it could scarcely be anything but the larva of Stylocidaris affinis. 



