DEVELOPMENT OF IRE ECHINOZOA. 



263 



larvae (Figs. 2 and 3). The young Urchin or Ophiurid gradually encroaches upon the Pluteus to 

 such an extent that it forms an essential part of the body, the arms and rods seeming to be mere 

 appendages, which ultimately disappear altogether. 



The development of the young Holothuriaii from its larva is much simpler than that of the other 

 Echinozoa. There is but one continuous longitudinal ciliated band around the bi- 

 lateral larva (Figs. 5, 6, c), instead of two, as in Bipinnaria ; and this does not throw 

 out lono- processes, but only becomes deeply sinuated. Certain parts of the sinuated 

 portions of opposite sides become united together, while others are obliterated, so 

 that the larva, which has become barrel-shaped, is surrounded by a number of trans- 

 verse ciliated rings. These in their turn disappear, the body of the larva elongates, 

 and tentacles appear round the mouth, while the water-tube usually loses its connection 

 with the exterior by the dorsal pore (Fig. 6, wp), and depends freely from the 

 water-vascular ring into the body-cavity, into which it opens. But the amount of 

 metamorphosis which the larva undergoes is not considerable, as it has no appendages 

 to be resorbed into the body of the adult. 



There is still less metamorphosis in the development of a Crinoid. The cilia, 

 which appear at first over the whole surface of the embryo, become restricted, before 

 it is hatched, to four transverse bands and a tuft at its hinder end (Fig. 7), while 

 the embryo becomes slightly curved, somewhat like a kidney-bean. In its concave 

 surface, which is turned downwards, is the single opening of its digestive canal, . 

 corresponding to the anus of a Bipinnaria. The larva gradually increases in length, 

 and delicate limestone plates make their appearance near its front end, arranged in 

 two cross-rings of five plates each. The plates of the lower ring, which are called the 

 basals, rest upon the top joint of a short stem, composed of delicate rings of limestone. 

 At this stage the larva has the form of a bent club or rod, with an enlarged head, 



which becomes the body of the future 

 Crinoid. The permanent mouth appears in 

 the centre of the upper ring of plates, which 

 are consequently termed the "orals." They 

 are gradually carried away from the cup 

 fcrmed by the basals by the appearance, 

 between the two rings of plates, of the 

 rudiments of the arms, which grow outwards as rapidly 

 elongating processes. The advanced Crinoid larva is known 

 as a " Peiitacrinoid " (Fig. 8, B), owing to its resemblance 

 to Pentacrinus* one of the Sea-lilies. The development of 

 a Crinoid is thus much more direct than that of a Starfish 

 or Urchin. There is no metamorphosis, and either the 

 whole of the larval body passes directly into the adult, or 

 the stem is discarded, and the cup with the arms attached 

 leads an independent existence. This is the case with the 

 Feather-star (Fig. 19), which separates itself from all but the 

 top joint of its stem, and anchors itself by little clawed hooks, 

 or cirri, that appear upon this joint (Fig. 8, B; Fig. 20, ci). 



A still more direct mode of development occurs among 

 most of the Echinozoa of the Southern and sub-arctic Seas, 

 T upper a ^d 1 th. 1 >ius at thefkfwCT radm emlieeted b^alfenrt which produce no free-swimming ciliated embryos at all. 



digestive tube, on the upper side of which is seen the nidi- _, - , ,. ,, .,, ..-, . ,1 



inrnrary water-vascular ring. The cilia are arranged in a But the VOUnff develop directly, either WltUlll Or Upon TJ16 

 sinuated band. (After Selenka.) * " * . . 



body of the parent, where they are protected until sufficiently 



advanced to look after themselves. In Urchins, Starfishes, and some Holothurians, the nursery or brood- 

 pouch is outside the body of the parent. In. some Urchins, for example, a kind of open tent is formed 

 in the neighbourhood of either mouth or anus by the approximation of two or three rows of spines. In 



* Greek, pente, five ; krinon, lily. 



4. 



VIEW OF THE AD- 

 VANCED HRACHIO- 

 LARIA LARVA OP 

 A STARFISH (As- 



terias vulgaris) . 



The whole of the ter- 

 minal anal part of the 

 larva has been ab- 

 sorbed into the disc 

 of the growing Star- 

 fish (r), the oral ex 

 treniity alone, with 

 its adjacent arms, 

 retaining its origin I 

 shape, ir, one of 

 the vaso-peritoneal 

 tubes, from which 

 the body-cavity and 

 the water-vascular 

 system are develop- 

 ed. \AfterA. Agassiz.) 



Fi 



5. LARVA OF HOLOTHURIA TrntJLosA IN ITS 

 NATURAL POSITION. 



