44 



MANUAL OF THE MOLLUSC A. 



hatclied, and they enter life under the same form, — that whioli 

 has been already referred to as permanently characteristic of 

 the pteropoda. (Fig. 69.) 



The Pulmonifera and Cepha'fpoda produce large eggs, con^ 

 taining sufficient nutriment to support the embryo until it has 

 attained considerable size and development ; thus, the newly- 

 born cuttle-fish has a shell half an inch 

 long, consisting of several layers, and the 

 hulimiLs ovatus has a shell an inch in. 

 length when hatched. (Fig. 31.) These 

 are said to undergo no transformation, 

 because their larval stage is concealed in 

 the egg. 



The researches of Joh7i Hunter f into 

 the embryonic condition of animals, led 

 him to the conclusion that each stage in 

 the development of the highest animals 

 corresponded to the permanent form of 

 some one of the inferior orders. This 

 grand generalisation has since been more exactly defined and 

 established by a larger induction of facts, some of which we 

 have already described, and may now be stated thus : — 



In the earliest period of existence all animals display one 

 uniform condition ; but after the first appearance of special 

 development, uniformity is only met with amongst the mem- 

 bers of the same primary division, and with each succeeding 

 step it is more and more restricted. From that first step, the 

 members of each primary group assume forms and pass through 

 phases which have no parallels, exce23t in the division to which 

 each belongs. The mammal exhibits no likeness, at any period, 

 to the adult mollusc, the insect, or the star-fish ; but only to 

 the ovarian stage of the invertebrata, and to more advanced 

 stages of the classes formed upon its own type. And so also 

 with the highest organised moUusca ; after their first stage they 



Fig. 31. 



labial tentacles; s s', the stomach ; b, branchise ; h, heart ; v, vent; I, liver; r, renal 

 organ ; a, anterior adductor; a', posterior adductor ; /, foot. The arrows indicate Uie 

 incurrent and excurrent openings ; between which the margins of the mantle are 

 united in the fry. 



* Egg and young of bidimus ovatus, Mlill. sp., Brazil, from specimens in the collec- 

 tion of Hugh Cuming, Esq. 



t " In his printed works the finest elements of system seem evermore to flit before 

 him, twice or thrice only to have been seized, and after a momentary detention to 

 have been again suffered to escape. At length, in the astonishing preparations for his 

 museum, he constructed it, for the scientific apprehension, out of the unspoken 

 fljphabet of nature."— Co/enc^^e. 



