LECTURES ON EMBRYOLOGY 



45 



come nearest to the Echinoderms. This arrange 

 ment, which is natural in itself, would show the 

 most admirable agreement between classification 

 &nd the phases of embryonic growth in this class, 

 and also because they come nearest to the first stage 

 of growth of the common Medusa. (Plate XIV 

 and XIX ] 



[SEE PLA.TES XIV LECTURE 3, AND XIX LEG 



TURK 4 1 



The analogy again between Medusa3 and Echi- 

 noderms is too easily ascertained to be ever mista- 

 ken by any one who attempts to compare them in 

 the same close manner. The chief difference here 

 consists in the more developed inner structure of 

 Echinoderms, whose organs are more diversified 

 and isolated, and in the harder coverings which 

 protect the soft parts, besides the addition of some 

 special apparatus which do not occur in the two 

 lower classes of Radiata. 



The improvements which I anticipate in the class 

 of Polypi are fewer, after removing the Retepora 

 and alied types, to the great groups of Mollusea 

 and the Tubulariae to the class of Medusa. We 

 shall only introduce the Porpite and Velella in the 

 vicinity of Actinia,and then, as Mr. Dana has done, 



6 



divide the Polypi proper in Actinoids and Alegon- 

 oids. the former division embracing those with, 

 simple tentacles, as Actinic, (Plate XX fig. D.) the 

 latter those ;vith fringed tantacles as Alegorium 

 and Renilla. {Plate XXXI.) 



All the stone corals proper belong to the type of 

 Actinia, and upon a close comparison of the struc- 

 ture of this animal with the ancient fossil Cyntho- 

 phyllum-like Polypi of palaeofoic rocks, some fur- 

 ther hints may be derived as to the order of sue 

 cession of Polypi in geological times, which is at 

 present very little understood. How the calcare- 

 ous stem is formed in Polyps, can be perhaps no- 

 where better studied than in the little Alcyonium 

 (Pate XXXI, figs. A, B,) of Boston harbor, where 

 calcareous nets and spicules are deposited in regu- 

 lar groups below and within the base of the tenta- 

 cles, and at the opposite extremities of the animal, 

 between which the muscular fibres are attached. 

 There is, moreover, a peculiarity in the structure 

 of Polypi, which can be easily observed in the 

 Ranella. (Plate XXXI, fig. K) In this Polype the 

 mouth has an elongated form, and there is one 

 tentacle in advance and one behind this opening, 

 in the longitudinal diameter of that fissure. 



Under tue form of radiated animals we have, in- 

 deed, through the classes of Echinoderms, Medusae 

 and Polypi, every where indications of a bilateral 

 symmetry, concealed under the more prominent 

 outlines of a radiated arrangement of the parts. 

 We have really among Radiata the first indica- 

 tions of the general bilateral symmetry which pre- 

 vails universally throughout the animal kingdom, 

 even in the class of Polypi. (Plate XXXIII, fig. 

 A.) In Actinia, the lowest condition, this bilateral 

 symmetry is noticed in the longitudinal direction 

 of the mouth, (Plate XX, fig. F) and in the ar- 

 rangement of the first formed tentacles, of which 

 one is seen always in the same diameter with the 

 mouth, whilst the other tentacles are placed in 

 two pairs on each side, (Plate XXXIII, fig. H) 

 which is peculiar in such species. We have also 

 indications of a bilateral arrangement in those 

 Medu^ie in which the body is compressed laterally 

 and more or less oblong, as in Beroe, Cestum, c. 

 where one diameter is much longer than the other. 

 We have it still further in the division of the ten- 

 tacles hanging down from the mouth in the com- 

 mon Medusa?, in which there is frequently one 

 tentacle more developed than the others. That 

 Echinoderms are regularly bilateral under their 

 spherical forms, I have already shown, fifteen 

 years ago, when I first ascertained that the Ma- 

 dreporia bodies lie always symmetrically between 

 two of their rays in the longitudinal axis, which is 

 parallel to the direction of the alimentary canal, 

 as It extends towards the elongated extremities of 

 the higher types of that class. 



Another peculiar arrangement which is common 

 to the Radiata, is the existence of water tubes, es- 

 tablishing a permanent connexion between the 

 surrounding element and the internal cavity of 

 the body. In the Medusa? (Plate XXVII,.figs. A 



