234 M. Sars on the Development of Star-fishes. 



2nd. The ova when laid (fig. 7) consist of a chorion investing 

 a small quantity of albumen, and the vitellus, which last soon ex- 

 hibits the usual jirocess of transverse division (figs. 8 — 10) now 

 ascertained to occur in most classes of animals ; they do not escape 

 forthwith into the sea, but are received into a kind of external 

 uterus formed by the parent voluntarily bending the ventral sur- 

 face of the disc and its arms, and which may be compared in 

 some respects with the pouch of the Marsupial Vertebrata. Here 

 the ova are hatched, and the young gliding from their interior 

 remain a considerable length of time, undergoing the progress of 

 development. This uterine receptacle is completely closed while 

 the ova are being deposited therein, and until the organs of at- 

 tachment of the young are perfectly developed. During the whole 

 of this time the mother can probably take no nourishment, since 

 the cavity being shut admits of no communication to the oral 

 aperture from without ; in this curved and contracted state (fig. 2) 

 the Star-fishes have been observed to rest immoveably in the 

 same spot for at least eleven days. A truly remarkable example 

 this of the care bestowed upon their young by animals otherwise 

 upon the lowest grade of organization ! 



l_Obs. — We are acquainted among the lower animals with several 

 examples of a kind of incubation being requned by the ova in 

 order that they may attain their development. Thus in the Me- 

 dusa the ova pass out from the ovaria into the pockets formed 

 by the four large oral caeca ; in the freshwater Mollusks, as Unio, 

 Anodonta, into the external branchial lamellae ; in the Crustacea 

 to beneath the belly or tail, in order to be submitted for a cer- 

 tain time in these situations* to the maternal influences. There is 

 however, as far as I am aware, no other example of a uterine cavity 

 being formed voluntarily by the mother on the outside of her own 

 body, and in this respect the instinct of the Star -fishes is indeed 

 unique. The circumstance of the Star-fishes taking no nourish- 

 ment during the incubation of their ova, finds its analogue in the 

 similar behaviour of several other animals, e. g. in the Serpents, 

 according to the observations of Valenciennes, who records an 

 instance of a Python that fasted fifty-six days while engaged in 

 cherishing its eggs.] 



3rd. The whole of the vitellus becomes converted into the 

 foetus. The latter, upon escaping from the ovum, has an oval 

 cylindrical form (fig. 11), is destitute of external organs, and 

 swims about freely in the water by means of numerous cilia 

 covering the body, like the Infusoria or newly-hatched young of 

 Medusa, Corynea, Alcyonia, &c., which it very much resembles 



* According to Joly (Mem. sur la Caridina Desmarestii in Ann. des Sci. 

 Nat. 1843, p. Gl), the eggs of the Crustacea cannot be withdrawn from the 

 mother without perishing. 



