276 LECTURE XIII. 



new animal may be said to creep out of the old, from which the pro- 

 cess is called the "ecdysis." With regard to the so-called metamor- 

 phosis which issues in the succession of a fixed, blind, sessile, multi- 

 valve barnacle to a free-swiraming crustacean with pedunculated eyes, 

 or in the succession of a rooted vermiform parasite to a natatory 

 animal with articulated setigerous limbs, — when these phenomena 

 are closely traced, they are seen to depend in a greater degree upon 

 the action and coalescence of retained cells, than upon a change of 

 form of pre-existing tissues. If the development of the ovum in the 

 pedunculate ovarian sac of the low crustaceous external parasite of a 

 fish be closely traced, the peripheral cells of the germ-mass are seen 

 to combine and coalesce to form the smooth transparent skin of the 

 embryo Lernsea, from which also tubular processes extend in two 

 (^Achthei^es) or three {Lernceocera) pairs, including setce v/hich pro- 

 ject from their extremities. 



In the LerncEocera the anterior pair is directed forward like an- 

 tenna3, but they are unjointed ; and the head is further indicated by 

 a coloured eye-speck. Another layer of germ-cells have perished, 

 as such, in order to form the parietes of a straight and simple 

 intestine, with a mouth and anus. Thus the anellidous type is first 

 manifested. 



But a large proportion of the minute germ-cells remain in the wide 

 abdominal interspace, amassed around the alimentary tube, and ag- 

 gregated in groups at the base of the tubular and setigerous feet. 

 With respect to the latter, we might say that the same provision is 

 made for the reproduction of the limbs as is retained throughout life 

 in regard to those of the lobster. In the larval Lernnsa, however, 

 those reserve-cells commence the formation of new limbs irrespective 

 of any injury to the old ones. The whole peripheral stratum of the 

 retained germ-mass, in contact with the primary integument, is trans- 

 formed into a new integument. These germ-cells have inci*eased 

 and propagated at the expense of the aliment assimilated by the 

 alimentary canal. The formation of the new integument and of the 

 new feet proceeds connectedly and contemporaneously; but tlie new 

 parts are not moulded upon the inner surface of the old ones. The 

 plastic force has changed its course of operation. A hinder segment 

 of the body is added to the front one, which answers to the whole of 

 the body of the first larva. If antennas did not before exist, a joined 

 pair is now developed. Instead of two pairs of tubular setigerous 

 limbs, three pairs of uncinated prehensile limbs are developed from 

 the anterior or cephalothoracic segment, and as many pairs of arti- 

 culated setigerous limbs from the abdominal segment. New muscles, 

 new nerves, and new vessels are formed for the support and exercise 



