LECTURES ON EMBRYOLOGY. 



tacies corresponding to the central alimentary 

 tube of the Medusae, which is only drawn in be- 

 tween the gelatinous walls of the disc, though it 

 remains equally free as in Tubularia. The upper 

 cavity of Tubularia answers to the disc of Medusas 

 proper with its cavities ; and in both the ovaries 

 are outside of the alimentary cavity, as well as of 

 the main cavity of the body Indeed, the agree- 

 ment is perfect in every respect, and we must 

 come to the conclusion that from their structure 

 Medusas and Tubularia must belong to the same 

 class, Tubularia being Medusa with a stem, and 

 bearing the same relation to free Medusas, as cri- 

 noids bear to free starflshes. And so we have in 

 the class of Medusas attached types, as well as in 

 that of Echinoderms, and in that of Polypi. 



In a more general point of view, we may, how- 

 ever, compare further, all radiated animals, when 

 we shall find that they really constitute a natural, 

 well circumscribed group in the animal kingdom 

 agreeing in all important points of their structure 

 being strictly constructed upon the same plan, al- 

 though the three classes which we refer to this 

 great department differ in the manner in which 

 the plan is carried out. In the first place, I may 

 mention that besides Polypi, Medusae and Echi- 

 noderms, the other classes which were referred to 

 the type of Radiata, have been removed from it, 

 or are to be removed from this connection. The 

 intestinal worms indeed are truly articulated ani- 

 mals in tbeir fundamental plan of structure, and 

 have to be connected with the worms proper, 

 while the Infusoria, Polygastrica and Rotatoria 

 are very heterogeneous classes, the latter of which 

 has to be united with the Crustacea and the so- 

 called Polygastrica, to be divided off according to 

 their various structures, some being germs of 

 aquatic plants, and others the first stages of growth 

 of various worms, as I have ascertained by direct 

 observation. As for the classes of Polypi, Medusae 

 and Echinoderms, if we bring together the dia- 

 grams (Plate XXXII) representing an Actinia in 

 a vertical section, with that of Plate XXXYII, 

 which represents a similar section of a Medusa, 



[PLATE XXXVII MEDUSA.! 



vert the Polype and place it with the mouth 

 downwards, as it is naturally in free Medusae, 

 we could see at once that in the Polypus we 



XXXVTU 



and other illustrations of Echinoderms exhibited 

 in a former lecture, and the vertical section of an 

 Echinarachnius, we shall have the elements 

 (Plate XXXVIII, fig. E) of a closer comparison be- 

 tween the three classes. If we were indeed to in- 



have lUc sumo -i.-n-^iU arrdnjfcitient as in the 

 dusae. There being a separate alimentary cavity 

 and a common cavity of the body only in Medu- 

 sae, (Plate XXXVII) the anterior part of the ali- 

 mentary cavity hangs down with the mouth freely 

 from the walls of the body. This part of the ali- 

 mentary canal answers to the cavity of Actinia 

 (Plate XXXII. f\s B) which is called stomach, and 

 from the upper part of the Actinia, in its inverted 

 position, arise those partitions which end in ten- 

 tacles answering to the disc of Medusae, with its 

 cavity, branching into similar tentacles. 



We have also again a common cavity in Medu- 

 sas (Plate XXXVII), as well as in Actiniae, only 

 more circumscribed, and branching off into tubes 

 which communicates in similar mannerwith the ten- 

 tacles, so that the general arrangement is perfectly 

 identical. The difference is, however, this that in 

 Medusae the tubes arising from the central cavity 

 are circumscribed, while in the Actinire (Plate 

 XXXII, fig. B) they are only partitions communi- 

 cating all together. And in the Medusae (Plate 

 XXXVII) there is a distinct nervous system. I 

 suspect that in Polypes we should find the nervous 

 system in the same position as a ring round the 

 mouth, if it is at all distinct in those animals ; that 

 however eye-like specks have been noticed, even 

 in these lowest animals, I have already mentioned. 

 As for the ovaries of the Medusas (Plate XXXVII), 



