MUSEUM OF COMPARATIVE ZOOLOGY. 137 



Formation of the Young Echinarachnius. 



The growth of the young Echinarachnius from its pluteus is not easy 

 to trace on account of the condensation of pigment upon its walls as it 

 matures. This formation of pigment renders it very difficult to study 

 the sequence of the appearance of the plates, and obscures the internal 

 changes which accompany the maturation of the larva into the adult. 

 The contour of the young sand-dollar after it absorbs the pluteus is very 

 different from that of the adult. No one would recognize both as be- 

 longing to one and the same Echinoid. The whole of the pluteus is 

 absorbed into the growing Echinarachnius. 



A vesicle, the vaso-peritoneal vesicle, on the left hand side of the 

 stomach (see figures) appears in the very earliest stages of the growth 

 of the sea-urchin from the pluteus to enlarge, and was observed to have 

 the form of a retort-shaped structure, with an external opening on the 

 dorsal side of the body, near the posterior arms, PI. VII. fig. 3. It was 

 not possible for me to determine whether the left " water-tube " sends 

 out a prolongation which forces its way to the surface, opening through 

 a dorsal pore, as A. Agassiz has described in Strongylocentrotus, or 

 not. In the earliest stage in which I began to study the growth of the 

 young sand-dollar, the dorsal opening had already formed, communi- 

 cating through a tubular body with the water-tube. Consequently, 

 the growth of the tube through the body was not observed or studied. 

 In the pluteus in which this external opening had formed, the arms of 

 the pluteus were all of the same length, and consequently the pluteus 

 was regarded as adult. In the pluteus of Strongylocentrotus, accord- 

 ing to A.. Agassiz, the young sea-urchin first appears in a young or im- 

 mature pluteus, in which the arms are not of the same length, judging 

 from his fig. 52, in "Revision of the Echini, Embryology," p. 717. In 

 this figure the autero-internal arms had not begun to push out from 

 the oral lobe, and the antero-lateral rods were just formed. This plu- 

 teus appears to be immature as far as the appendages go, since they 

 are not fully formed. The beginning of the young Echinarachnius on 

 the left water-tube was not traced in a pluteus as young as this pluteus 

 of Strongylocentrotus. 



Balfour* in his account of this figure (fig. 52) gives an interpretation 

 to the structure, t, difiereut from A. Agassiz. The latter author says, 

 *' On the left water-tube we notice a very prominent loop, t, which, from 



♦ Op. cit. pp. 472, 473. 



