14 



PROF. AGASS1Z 7 S 



These figures [Plates III and IVj represent the 

 changes which I have observed in a species of 

 Star-fish from Boston harbor, from its first forma- 

 tion in the egg up to its perfect condition ; though 

 I have not been able to trace it to the full size to 

 which it grows on these shores. Sars has not 

 been able to ascertain the internal structure of the 

 Star-fish, because the species which he observed 

 was too opaque, and did not allow an investigation 

 of the internal parts. The species which I have 

 compared admitted of such an examination, hav- 

 ing more transparent parts, and by a peculiar pro- 

 cess of investigation it has been possible to ob- 

 serve the whole internal structure, the specimens 

 being pressed between two glass plates, when 

 placed under the microscope. 



Before I allude to alj the details represented in 

 plate III & IV, let me show from these figures how 

 I conceive that the diagrams of Sars [Plate II] 

 though drawn from nature, give an erroneous im- 

 pression of the animal. It is simply that the pe- 

 duncle hanging from the centre of the discoid or 

 spherical body being laid flat upon a glass plate, 

 and perhaps pressed it on the glass, for the mi- 

 croscope is bent sideways, and thus it is seen as 

 in these figures. But when seen floating, it will be 

 noticed that this peduncle hangs downward, [Plate 

 III, fig. A, B, C, D]. 



As a class of animals the Echinoderms agree most 

 remarkably in their structure, though differing 

 most widely in their external forms. We have in 

 the first place elongated forms, somewhat like 

 worms, with 'a star- shaped extremitj', called Ho- 

 lothurise. 



[PLATE V HOLOTHUT?T^E ] 



[PLATE VI ECHINODERMATA J 



Here are spherical or spheroidal forms of these 

 animals called Echini or Sea-Urchins, [Plate VI] 

 and finally star-shaped ones, called star-fishes, 

 and among which there are free ones, those which 

 rest on a stem, like lilies, [Plate VII. fig. A D ] 

 PLATE VH STAK-FISHES CRTNOIT>= 1 



These various animals are so widely different that 

 it seems scarcely possible to find a fundamental 

 plan of structure and a uniform arrangement of 

 parts in all of them. Yet it is so. Conceive for a mo- 

 ment that the fundamental form is a spherical one. 

 K the sphere is extensively elongated, we have the 

 form of the Holothurise, Plate V. ; the spheroid 

 form itself may be more or less ovate [Plate 

 VI. I or angular; or if the corners of these be 

 drawn out, we have a real star-fish. In the cen- 

 tre of some of the circular ones there are plates 

 or prominent knobs on the summit, [Plate I. fig. 

 B.] which may form a kind of peduncle above. 

 Now it is easy to conceive that these growing 

 longer will appear in the shape of a longer or 

 shorter stem upon which the animal will move, 

 [Plate VI. fig. A DJ balancing itself. So that from 

 these polypi-like forms up to the worm like forms 

 we have gradual transitions. 



As the highest among the radiata the echino- 

 derms are more complicated in their structure, 

 Their external coverings are already more distinct 

 than in any other. In the polypi the skin is close- 

 ly attached to the fleshy mass of the body. Here 



