LECTURES ON EMBRYOLOGY. 



31 



somewhat anticipated by Sars, by Sir John Daly- 

 'ell, find by a French naturalist. Da Jardin; though 

 they did not carry out their investigations to the 

 same purpose, yet they led the way in the same 

 track. How these changes take place, will be I 

 suppose better understood if I begin by giving an 

 outline of the structure of these animals, which it 

 has been possible for me to examine more com- 

 pletely than it had been done before; availing my- 

 self of several small species which liv-e in Boston 

 harbor. The large animals are not those which 

 are best suited to such investigations; when large 

 their bulk prevents their being isxami&ed under the 

 microscope. But let the animal be small enough 

 to be placed entire under the microscope and you 

 get a general view of the structure ; and by apply- 

 ing a higher power to the various parts, you can 

 trace the details in such a way as to ascertain 

 most completely their organisation. Such was the 

 process by which I was enabled to discover in 

 these minute medusas, even the nervous system, 

 which had been only suspected, but not traced in 

 its distribution. And let me add, that beside their 

 physiological interest, these animals are wonder- 

 ful in their aspect, and present the most attractive 

 .sight which can be witnessed. Their transparent, 

 delicate bodies swimming freely in the water and 

 moving regularly by the contraction of their 

 whole mass the elegance of their outline and the 

 diversity of the appendages which hang down from 

 their globular body or the suckers which rise from 

 the centre,, and constitute other appendages from 

 the middle of the sphere all these contribute to 

 make these animals wonderfully beautiful. An in- 

 creased interest is felt when seeing at 5rst scarcely 

 an outline, so transparent are they, and discovering 

 afterwards by Che simplest process of examination, 

 consisting in modifying the light which passes from 

 the mirror of the microscope through their body, 

 -all the differences of structure so easily overlook- 

 ed at first sight. 



And again, they belong to a class of which so 

 many are transparent, or pliosphorescent,that there 

 are endless inducements to investigate these ani- 

 mals. Here are various figures (Plates XXI, 'II, 

 III, 'IV, 'V, 'VI, 'VII), all representing Medusa. 



Many of these figures are of a hemispherical 

 form, as plate XXI ; and this form (Plate XXVII, 

 fig. B). In the margin of this form (Plate 

 XXI, tig- A) you see we have two kinds of appen 



[PLATE XXI MEDUSA.] 



[PLATE XXII MEDUSA.] 



(PLATK XXIII MBI>CA.J 



dages, and you see (tig. B) that there is a central 

 cavity, and that there are four bunches of a pecu- 

 liar character here, the ovaries, (fig. C), and that 

 the lower surface presents various rays diverging 

 towards the edge. In another form, Beroe, (Plate 

 XXII) we have a tubular hody with vertical rows 

 of vibrating cilia., and a wide opening below the 

 internal cavity, which is more complicated than 

 that of the odier types. Here is (Plate XXVI) an- 

 other, Agalcnopsis, which is still more complicated, 

 from the diversity of all the appendages which 

 hang from the main stock ; and here is another, 

 which is, if possible, still more complicated, and 

 has a very large vesicle above and numerous ten- 

 tacles hanging below. This animal (Plate XXIII) 

 is known to the sailors by the name of Portugese 

 Man-of-war. Naturalists call it Physalia. Others 

 are flat, circular, or oval, with several rows of sim- 

 ple appendages, as Velella, plate XXIV, and Por- 

 pita, plate XXV. 



[PLATE XXIV MEDUSA.] 



Esciischoit, who fius studied iliese 

 animals more extensively than any one else, has 

 divided them into three groups Ctenophora, Dis- 

 cophora, and Physophora, Those which have 

 these vesicles, by which they are suspended in the 

 water, are called Physophora. They are all con- 

 sidered as simple animals, though their form is 

 extremely complicated. Here is an enlarged fig- 



