222 Br. J. D. Macdonald on tlie Classification 



Tunicata, we finally enter the precincts of the Mollusca 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 type 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 morpho- 

 logical resources of the Molluscan type would ajipear to be 

 ample enough*. 



2. The Gregarinidas evidently hold a superior position to 

 the other astomatous families of Protozoa ; and it would appear 

 as though the cestoid Entozoa were derived from them. In 

 this connexion the sucker occurring in Actinocephalus and the 

 circlet of uncini like those of TcEnia 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. TetrarTiynchus jlavice])s and 

 such astomatous forms have probably led the way to the more 

 highly organized Acanthocephala and the Nematoidea gene- 

 rally, to which may have succeeded the Trematoda. 



3. Many Infusoria have been taken for the larvae of Tur- 

 bellaria, and vice versa, giving some support to the view that 

 the latter order of animals may have descended from the 

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

 ArliyncJda or Nemertes with that of 8ipunculus'\ , and study 

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

 be made apparent ; and if this be so, next must follow Syn~ 

 apta, Holothuria,' in fact the Echinodermata as a whole, to 



* A more detailed account of the moi'phological relations of the Mol- 

 liiscoida and Coelenterata will be found in a paper on this subject by the 

 author, published in the ' Transactions of the Royal Society of Edinburgh,' 

 18G4, vol. xxiii. part 3. 



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

 supraoesophaoeal ganglia in Si^nmndus, with a bilateral distribution 

 of nerves to the circlet of simple and finely ciliated tentacida. Moreover 

 I have fovmd an unequivocal eye-speck in connexion with each ganglion. 

 The ventral threads are plain and destitute of the series of ganglia occur- 

 ring in the Ilirudinea, to which group many naturalists refer the genus. 

 The intestine passes backwards, winding round a suspensory tendinous 

 cord, upon wliich it returns to the position of the dorsal anus ; and the 

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

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

 the integument consists of pointed tuberculations over the middle and 

 posterior regions of the body, and gradually approximating transverse 

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

 tentacula. 



