222 Dr. J. I). Macdonald on the Classification 



Tunicata, we finally enter the precincts of the MoUusca proper. 

 This is, no doubt, the " royal road " to the Vertebrata, if, in- 

 deed, there be any ; for it would be hopeless to seek for the 

 evolution of this higher ty[3e through any of the other channels 

 to be briefly noticed in this paper. The Insects, Crustaceans, 

 and Echinoderms present impassable barriers in this respect. 

 They are so curiously constructed, exhausting one's ideas of 

 modification, or so perfect in their way as to preclude any 

 conception of their further development ; but the morjjho- 

 logical resources of the Molluscan t^-pe would appear to be 

 ample enough*. 



2. The Gregarinidic evidently hold a superior position to 

 the other astomatous families of Protozoa ; and it woidd appear 

 as though the cestoid Entozoa Avere derived from them. In 

 this connexion the sucker occurring in ActinocejJialus and the 

 circlet of imcini like those of Tmnia in Hoplorhynchus are 

 very significant. Moreover the usually elongated body of 

 these animals is invested with a more distinct tegumentary 

 coat than that of the Rhizopoda. TetrarJii/nchusfiavicejjs and 

 such astomatous forms have probably led the way to the more 

 highly organized Acanthocephala and the Xematoidea gene- 

 rally, to which may have succeeded the Trematoda. 



3. ]\Iany Infusoria have been taken for the larva3 of Tur- 

 bellaria, and vice ve/sd, gi\dng some support to the view that 

 the latter order of animals may have descended fi-om the 

 former. And further, if we compare the internal anatomy of 

 Arhjjnchia or Nemertes with that of Si])tinculiis'\, and study 

 also the larval state of these animals, their close affinity will 

 be made apparent ; and if this be so, next must follow Si/n~ 

 upta, Ilolothuria, in fact the Echiuodermata as a whole, to 



* A more detailed account of the moi-phologrical relations of the Mol- 

 luscoida and Cwlonterata will be found in a paper on this subiect by the 

 author, published in the ' Transactions of the Koyal Society of £diubui*gh,' 

 18G4, vol. xxiii. part .'}. 



t I can confirm the statement of Krohn as to the existence of two 

 supraa'sophapeal paujilia in >Sipmicu/u,-<. with a bilateral distribution 

 of nerves to the circK't (>f simple and tint>ly ciliated teutacula. Moreover 

 I have found an unequivocal eye-speck in connexion with each franglion. 

 The ventral thn^ids are plain and destitute of the series of ganglia occur- 

 ring in the Ilirudini'a. to which group many naturalists refer the genus. 

 The intestine passes backwards, winding round a suspensory tendinous 

 cord, upon wliifii it returns to the position of the doi"sal anus; and the 

 perivisceral lueinbrane is richly ciliated, forming little mesenteries to in- 

 close the intestiniil vessels. In the coral-boring species the armature of 

 the integumi'ut consists of pointed tubercnlations over the middle and 

 posterior regions of (lie body, and gradually approximating transverse 

 rings of recurved hooks ixteuding along the fore part to the base of the 

 tentacula. 



