476 CLASS II. CILIATA. 



Coelenterata, including the Corals, Sea Anemones, and Hydrozoa, the initial term 

 or larval form common to the group is an ovate, evenly ciliated body, or so-called 

 " Planula," woodcut, Fig. i , differing entirely from the parent organism, but in all ways 

 comparable with a mouthless Holotrichous Infusorium, as delineated at Fig. 2. In a 

 similar manner, the mostly endoparasitic Scolecida (Trematoda and Cestoidea) are 

 traceable to a like origin, many Trematodes, such as Monostomum, giving birth to 

 simple holotrichously ciliated embryos, while the aberrant Opalinidae, with reference 

 to their often highly differentiated organs of adherence and, in certain instances, 

 e. g. Anoplophrya prolifera, peculiar mode of reproduction, indicate a no very distant 

 connection with the Cestoidea. It is worthy of remark that while many of the higher 

 Annelids, Lumbricus and JVais, commence life as similar holotrichous infusorium-like 

 embryos, others, including the majority of the marine worms or Polychaeta, conform 

 for the most part, as do the larvae of the Echinodermata, with the Peritrichous 

 infusorial formula. 



The order of the Heterotricha, whose members unite, in their plan of ciliation, 

 both the Holotricha and Peritricha, do not at present suggest any distinct Metazoic 

 affinities. It is at the same time perhaps deserving of attention that the ciliated 

 lappet-like appendages developed in the larval stage, or so-called " Pilidium " of the 

 Turbellarian genus Lineus, recall to some extent the ciliated lobes of the peristome 

 developed in the Heterotrichous genus Follicularia. 



While, in the preceding volume, loc. cit. p. 103, a passing reference was made to the 

 affinities of the Peritrichous Vorticellidae with the Polyzoa, suggested by both L. 

 Agassiz and K. M. Diesing, such faint external likeness as subsists between the 

 adult structures and gave rise to such correlation, was not considered worthy of 

 serious entertainment. Since then, however, it has occurred to the present author that 

 from an entirely diverse standpoint the relationship suggested may be substantially 

 upheld. A first impression in this direction was derived from a recognition of the 

 close correspondence in external contour presented by the motile larvae or so-called 

 Trochospheres of the Polyzoon Alcyonidium gelatinosum, as delineated in a notebook 

 placed at the author's disposal by Mr. H. E. Forrest, and reproduced at PI. XXXI. 

 Fig. 53, and such a free Peritrichous Infusorium as Trichodina pediculus. Following 

 out this newly indicated clue, it was next found that the larval conditions of 

 numerous other Polyzoa, as recently figured and described by Barrois,* bear out in a 

 most remarkable manner the premised affinity. This last-named authority, further- 

 more, reduces all of the more or less modified embryos of this zoologic group to an 

 ideal type, see Fig. 1 1 of the accompanying woodcut, which, without a pre-existing 

 knowledge of its true import, would unhesitatingly be accepted as a slightly modified 

 example of the ordinary Vorticellidae, as given at Fig. 12. It is highly interesting 

 to find that the class Mollusca, whose intimate relationship with the Polyzoa is 

 universally conceded, abounds with larval forms reducible to the same Peritrichous 

 formula. A suitable illustration of this fact is afforded by the figures representing the 

 early larval condition of Chiton anereus, as delineated by Lov^n, and reproduced in 

 Huxley's ' Manual of the Invertebrata,' and in other zoological textbooks. 



Yet another leading section of the Metazoa, that of the Echinodermata, would 

 appear to admit of being retraced phylogenetically to the group of the Peritricha. 

 Here, as in the preceding instances, the clue has to be sought among the larval 

 or embryonic, and not among the adult organisms. The embryo Echinoderm, or so- 

 called Echinopaedium (Huxley), on leaving the egg, is altogether distinct from the 

 parent, consisting of a laterally symmetrical, ovate body having usually two, four, 

 or more girdle-like bands of cilia. Through the process of histolysis or atrophy, this 

 primary integument with its ciliary girdles becomes speedily obliterated, but there 

 can be no doubt that during such transient larval existence the young Echinoderm 

 resembles in a truly remarkable manner such Peritrichous Ciliate Infusoria as Urocen- 

 trum turbo, PL XXXIII. Figs. 7-9, and Didinium nasutum, PL XXXII. Figs. 50-57, 

 and woodcut, Fig. 10, the latter type more especially being characterized in its normal 



* ' Recherches sur 1'embryologie des Bryozoaires,' 1877. 



