LECTURES ON EMBRYOLOGY. 



cics, toe position of the heart can be recognized by 

 definite outlines. These outlines in the shell 

 (Piate XVII, fig. A) cover the position of the heart. 

 This other outline indicates the position of the gills. 

 This becomes possible, from the fact that those or- 

 gans, though soft, are earlier developed than the 

 shell, which is modeled over the organs. 



The position of the gills is important on one ac- 

 acount, being always connected with the legs ; 

 though they appear to differ widely in their posi- 

 tion in various Crustacea. Where they are cover- 

 ed, they are attached upon the thieb, the shield 

 extending over their point of insertion. 



In other Crustacea, the gills are external they 

 are attached to the external joints of the legs, and 

 seem to present an entirely different connection 

 from what we observe in the Crabs, But the mo- 

 ment we go to the bottom of the question, we see 

 that here also the respiratory organs are connect- 

 ed with the ieg, only that they are from the upper 

 portion, and covered by a shield, as it is devel- 

 oped. 



In those Crabs, the nervous system presents a 

 very interesting arrangement Above the alimen- 

 tary canal there is a first mass, which gives threads 

 for the head proper, a kind of brain; a ring around 

 the alimentary canal connects this swelling with 

 ihf other swellings; but these posterior swellings 

 form only one uniform mass in the centre, from 

 which threads go to all the rings of the chest and 

 their appendages. And this position, this concen- 

 trated arrangement of the nervous mass, is observ- 

 ed in all Crabs. In other Crustacea, the nervous 

 centres under the alimentary canal are more or 

 less scattered, and correspond directly in their po- 

 sition to the rings which they furnish with nerves. 

 This structure of the nervous system plainly shows 

 that the Crabs must be considered as ranking 

 highest among Crustacea, if we remember whajt 

 has been observed in the Caterpillar, in which, 

 during its metamorphosis into a higher form, the 

 nervous swellings were observed to concentrate 

 gradually more and more into compacter and 

 fewer masse?. [PLATE VI page 41 LOBSTER.] 



Let us now compare these Crabs with a Lobster, 

 (Plate yi) or with a Shrimp, (Plate XVII, fig. C) a 

 species of Shrimp which occurs in the Southern 

 States, called Peneus setifer. The general arrange- 

 ment of the parts is the same as in Crabs. Here 

 we see irst. the cephalo-thorax covering the main 

 organs, and the anterior pairs of legs, covering 

 also the mouth, and from which, on the anterior 

 part, arises the peduncle for the eye and those 

 appendages called the palpi. Next, we distinguish 

 the tail, which is continuous with the head-chest, 

 and forms a large part of the body ; a portion of 

 the body, which is as large as the cephalo-thorax, 

 or even larger, and which can be curved forwards, 

 but which is never permanently bent under the 

 cephalo thorax. Such an arrangement of parts is 

 also observed in the Lobster, (Plate VI) which 

 does uot differ materially in its structure from 



what we have noticed in the Shrimp. The various 

 rings which constitute this cephalo-thorax and the 

 tail, are equally provided with moveable appen- 

 daeres, which are represented separate in Plate 

 XXL In the head we notice a short peduncle, 

 (Fig. A) terminating with a compound eye,consist- 

 ing of thousands of little lenses, each of which has 

 a crystaline lens, a nervous thread, and really is 

 a compound eye. Next, we have those two sorts 

 of palpi represented in figs. B, C. Next we have 

 six pairs of horizontal moveable jaws, (Figs. D, I,) 

 three of which are more powerful perhaps than 

 the others, and constitute what are called the jaws 

 (Figs. D, F); whilst the three others are called jaw- 

 feet, from their close resemblance to the legs in 

 many of these animals. 



[PLATE XXI APPENDAGES or CRUSTACEA.] 



The three first pairs, which are near the palpi 

 are properly called jaws; and the three following 

 pairs are called jaw-feet, (G, H, I). They are call- 

 ed jaw-feet, for having internally, like the legs 

 proper, appendages which are modifications of the 

 apparatus which supports the gills proper. These 

 appendages, however, (Figs. G, H, L) instead of 

 being complicated gills, have only fringed mem- 

 branes, extending backwards, without performing 

 respiratory functions. So that, in these parts which 

 surround the mouth and act as jaws, we have the 

 same connection between the respiratory organs, 

 as that we observe in the legs under the chest. 



