LECTURES ON EMBRYOLOGY. 



65 



XVIII, figs. A, B, and C), we have a structure re- 

 sembling the Shrimp in its general outlines ; but 

 in the eye, we have no longer a peduncle. The 

 eye is sessile that is to say, it does not rise above 

 the surface of the body upon a peduncle. 



In the others, Decapoda and Stomapoda, the 

 eyes proceed from a moving peduncle, and are 

 provided with the peculiar apparatus for seeing. 

 Such eyes are, therefore, moveable upon the joints 

 of the peduncle; but in these Amphipoda the eyes 

 are flat upon the shield (Plate XVIII, fig. B). You 

 see that there is a diversity of legs among them, 

 and a peculiar kind of claws in the anterior part- 

 various appendages performing at the same time 

 the function of legs and gills, and the tail similiar 

 to that of Decapoda (Plate XVII, fig. B). One 

 modification, however, will strike you. There are 

 no longer many joints united to form a cephalo- 

 thorax, but all the joints are nearly equal. The 

 head constitutes only a joint similar to those of 

 the rest of the body. There is no concentra- 

 tion of legs in distinct regions. The number of 

 these animals which occur in this vicinity is very 

 great 1 ; but they have, by far, not all been described- 

 A few only have been mentioned in Dr. Gould's Re- 

 port. Even genera which have not been described 

 at all, occur in the harbor of Boston. Here, for 

 instance (Plate XVIII, fig. C), is one of the new 

 species, a new generic type, which is very beauti- 

 ful. It is a curious fact that among these animals 

 there is such a variation of color. I have had a 

 good many of them drawn and painted, in order to 

 collect all the variatioas of colorations which exist. 



It is scarcely possible to find two specimens which 

 acree in color ; and many differ in the distribution 

 of color so much, that if they were brought from 

 different countries, and if it was not known that 

 they lived together, Naturalists might arrange them 

 as different species. In various individuals of the 

 same species, (Plate XVIII fig. A) we find some 

 are red, and others (Fig. B) green, others bluish, 

 and- others still, with every variety of color. To j 

 this fact I shall call again your attention hereafter. 



We have (Fig. E) others still different, in which 

 the different joints are so slender as to form an 

 elongated figure with outward appendages to it. 

 The middle appendages are very simple; the anterior 

 ones have claws, while the posterior ones are mere 

 simple legs. But on the whole, they come near to 

 the Amphipoda, (Plate XVIII, fig. A.) As the 

 legs, however, show some modified combinations, 

 they have been considered as a peculiar family, 

 under the name of Loemodipoda. 



In some Crustacea of another form, (Plate XVIII 

 fig. D) the rings are also not combined in distinct 

 regions, and the eyes arise equally from the level 

 surface of the shield ; but the legs are uniform, and 

 the uniformity goes on, increasing as we proceed 

 lower down, to the various forms of this type 

 which comprise the Isopoda. 



All the Crustacea of which I have spoken, have 

 one common character a thin calcareous shell : 



[PLATE I GERMS OF SCORPION.] 



whence their common name of Malacostraca is de- 

 rived. Those of which I am now to speak are dif- 

 ferent in this respect, and have been called Ento- 

 mostraca. Some of them (Plate XVIII, figs. G and 

 H, and Plate XX, figs. F and L,) are Parasitic 

 Animals, in which we observe two long ap- 

 pendages, or ovaries, hanging down from the 

 posterior joints. The body in the Entomos- 

 traca is simply protected by a horny shield or 

 envelope, lining the back. There are some (Fig. 

 G) in which the body is elongated, in the shape of 

 a Worm, and in which the joints are almost en- 

 tirely gone; so much do they differ from the com- 

 mon character of Crustacea; and indeed,in such an 

 animal as the Lernea, (Plate XX, fig. L) there is 

 no joint at all to be distinguished; there are not 

 even gills to be observed ; there are no legs to be 

 found in any part of the body ; there is no heart ; 

 no one of the leading anatomical characters of this 

 class of animals is observed in the Lernea; and 

 nevertheless it is a Crustacean. It is one of those 

 Crustacea which have been long known in 

 their later condition of life, when they have be- 

 come attached Parasites, but which have not been 

 known in their earliest stages of life, when they 

 are free, moving, independent individuals, with all 

 the characteristics of other Entomostraca and 

 similar Crustacea. These young, however, have 

 the structure of Crustacea, inasmuch as they have 

 fringes, appendages to their rings; inasmuch as 

 there is a nervous system, presenting the arrange- 

 ment of the nervous system in the Cyclops. But, 

 when they have been freed fora certain time, they 

 become attached, and are then Parasites, and un- 

 dergo a most remarkable retrograde metamorpho- 

 sis, by which they lose all the peculiarities of their 

 structure, sink to a lower condition of life, and 

 producing a great number of eggs in this condi- 

 tion, finally die by a peculiar kind of bodily de- 

 cay, as it were, which we nevertheless cannot con- 

 sider as a decay, as it is ''in this curious stage of 

 these animals ^that their eggs are most rapidly 

 produced. It is really, as Rathke has considered it, 

 a true retrograde metamorphosis in after life. But 

 it is remarkable that there should be animals be- 

 longing to the class of Crustacea, which have so 

 entirely lost the aspect of Crustacea; which have 

 no one of their anatomical characters, and which, 

 nevertheless, belong to that class, as is shown by 

 their metamorphosis. 



