32 



PROF. AGASS1Z S 



[PLATE XXV-MEDUSA ] 



ure (Plate XXVI) of one of these animals, with 

 all various appendages tentacles, suckers, groups 

 of eggs, and all sorts of vesicles forming one 

 elongated body, with fringes. It is the Agalmopsis 

 of Sars. Let us now see what is the structure of 

 these animals. The internal cavity communicates 

 with the exterior by a broad open mouth, as yon 

 see in this sketch, or by bunches of tentacles which 

 terminate in little suckers, as you observe in this 

 figure (Plate XXVII, fig. B) numerous suckers 

 hanging down from the central appendages and 

 forming as many mouths, as many openings com- 

 municating with the central cavity, as there are 

 such appendages. In another case (Plate XXVII 

 fig. A) we have simple little openings, or pores, up- 

 on the surface of the larger appendages, all direct- 

 ed inwards, uniting and combining to form larger 

 stems finally combining into fewer tubes and 

 emptying into a main cavity, and from that main 

 cavity branching off again into numerous tubes, 

 and dividing over the margin of the disc Those 

 ramifications from the central cavity towards the 

 surface can be easily seen by holding the light in a 

 certain angle before these animals in their living 

 condition. And by injecting colored water, you 

 may fill them in all directions, and see that there 

 is, as in plate XXVII, fig. A, a net work of vessels 

 ramified around the animal. 



There are others, [Plate XIX., fig M.] in which 

 there are main stems, which divide into some few 

 more towards the margin, or unite again into at 

 circular canal all around the edge. 



There are even some in which the central cavity 

 IPlate XXVIILj is very small, having only a little 

 sack at the summit of a long proboseis,whicb i&fihe 



mouth ; the little sack next divides into four tabes, 

 which then extend towards the edge, where they 

 unite again to form a circular tube. Liquids are 

 constantly circulated in these cavities. The food 

 is digested within that cavity and then circulated 

 through the tubes, and in those which have only 

 minute pores as oral apertures, the food can con- 

 sist only of microscopic animals, or of decomposed 

 organic matters in others which have a larger 

 mouth, larger animals are introduced In the 

 small species of Boston harbor, |Plate XXVII. fig. 

 C.j which was first described by Dr. Gould in his 

 Report upon Invertebrate Animals of Massachu- 

 setts, and which will be exceedingly common in a 

 a few weeks, I have seen this pioboscis hanging 

 down and stretched three times the length wnich 

 you see here; and after it had swallowed some- 

 thing, and the food had been digested, the globules 

 arising from the digestion would be circulated 

 through the tubes and would be seen under the mi- 

 croscope most plainly, diverging towards the mar- 

 gin of the sphere, there moving into the circular 

 tubes, or perhaps even moving down into the ap- 

 pendages those hanging arms which are hollow 

 and again trace back their course into the circular 

 tubes - T some of the globules would disappear wheu 

 absorbed by the surface, but the remainder is cir- 

 culated forwards and backwards to and fro in 

 those tubes before disappearing entirely. 



Such a structure can be considered the lowest 

 condition of a system of circulation, which is at 

 the same time a modification of the alimentary 

 tube, where the stomach divides, and where the 

 divided stomach again unites into vessels into 

 common vessels, which branch in their turn. Here 

 we have ihe tubes uniting and then branching off 

 again, [Plate XXVII. fig A.J but in Fig. C there is 

 a distinct mouth and proboscis. 



The mass which forms the body in medusa i& 

 transparent and cellular. And then there are 

 distinct muscular fibres of two kinds, circular 

 ones around the whole disc, and radiating ones, 

 which form distinct bundles diverging from the 

 centre towards the periphery; in those medu- 

 sa which have four diverging alimentary tubes- 

 the main radiating muscular bundles alternate with 

 the tubes (Plate XXVII, fig. C.) All these muscular 

 bundles and the circular fibres contract alternate- 

 ly, so that the body can be shortened or flattened 

 in various ways, and thus, through the agency of 

 these muscles, the animal moves in all directions, 

 upwards, sideways and downwards, at will. That 

 these animals moved by contraction, had long been 

 observed ; but the existence of regularly arranged 

 muscular fibres in the class of medusse, was still 

 doubtfal When Ehrenberg published his investi- 

 gations upon the structure of the medusae of the 

 Northern Seas, though he concluded that there 

 must be muscular fibres, he could not discover a 

 regular, complete, muscular system. However, in 

 these small medusse, the muscular fibres ar 



