488 OF ECHINODEKMATA. 



larvae is that which has received the name of Bipinnaria (Fig. 

 243), from the symmetrical arrangement of its natatory organs. 

 The mouth (a), which opens in the middle of a transverse fur- 

 row, leads through an oesophagus V to a large stomach, around 

 which the body of a Star-fish is developing itself ; and on one 

 side of this mouth is observed the intestinal tube and anus (5). 

 On either side of the anterior portion of the body, are six or 

 more narrow fin-like appendages, which are fringed with cilia ; 

 and the posterior part of the body is prolonged into a sort of 

 pedicle, bilobed towards its extremity, which also is covered 

 with cilia. The organization of this larva seems completed, and 

 its movements through the water are very active, before the 

 mass at its anterior extremity presents anything of the aspect of 

 the Star-fish ; in this respect corresponding with the movements 

 of the "pluteus" of the Echinida ( 322). The temporary 

 mouth of the larva does not remain as the permanent mouth of 

 the Star-fish ; for the oesophagus of the latter enters on what is 

 to become the dorsal side of its body, and the true mouth is sub- 

 sequently formed by the thinning away of the integument on its 

 ventral surface. The young Star-fish is separated from the 

 bipinnarian larva, by the forcible contractions of the connecting 

 pedicle, as soon as the calcareous consolidation of its integument 

 has taken place, and its true mouth has been formed, but long 

 before it has attained the adult condition ; and as its ulterior 

 development has not hitherto been observed in any instance, it 

 is not yet known what are the species in which this mode of 

 evolution prevails. The larva continues active for several days 

 after its detachment ; and it is possible, though perhaps scarcely 

 probable, that it may develope another Asteroid by a repetition 

 of this process of gemmation. 1 



322. In the Bipinnaria, as in other larva zooids of the Aste- 

 riada, there is no internal calcareous framework ; such a frame- 

 work, however, is found in the larvae of the Echinida and Ophi- 

 urida, of which the form delineated in Fig. 244 is an example. 2 

 The embryo issues from the ovum as soon as it has attained, by 

 the repeated segmentation of the yolk, the condition of the " mul- 

 berry mass ;" and the superficial cells of this are covered with 

 cilia, by whose agency it swims freely through the water. So 

 rapid are the early processes of development, that no more than 

 from twelve to twenty-four hours intervene between fecundation 



1 See the observations of Koren and Daniellsen (of Bergen) in the " Zoologiske Bid- 

 rag," Bergen, 1847 (translated in the " Ann. des Sci. Nat." 3e Se"r. Zool. torn, iii, p. 

 347) ; and the Memoir of Prof. Miiller, " Ueber die Larven und die Metamorphose der 

 Echinodermen," in " Abhaldlungen der Koniglichen Akademie der Wissenschaften 

 zu Berlin," 1848. 



2 See Prof. Muller, " Ueber die Larven und die Metamorphose der Ophiuren und 

 Seeigel," in " Abhaldlungen der Koniglichen Akademie der Wissenschaften zu Berlin," 

 1846. See also, for the earlier stages, a Memoir by M. Derbes, in " Ann. des Sci. Nat." 

 3e Se*r. Zool. torn, viii, p. 80; and for the later, Krohn's " Beitrag zur Eritwickelungs- 

 geschichte der Seeigillarven," Heidelberg, 1849, and his Memoir in " Muller's Archiv." 

 1851. 



