AFFINITIES 



31 



There are other points in which Balanoglossus specially resembles 

 Amphioxus, such as the early development, the mode of formation 

 of the body-cavities/ and the presence of numerous generative 

 organs. 



All these, taken together, make it necessary to consider 

 carefully the claims of Balanoglossus to relationship with the 

 ancestors of A'ertebrates in making any speculations on this 

 interesting problem. 



However improbable it may a.ppear at first sight, it is 

 possible to hold the view that Balanoglossus is related at the 

 same time to Vertebrates and to Starfishes and other Echino- 

 derms. The similarity between a young Tornaria and a young 

 Bipinnaria-larva of a Starfish is so great as to have misled even 

 Johannes Mliller. The more obvious resemblances are the almost 

 identical course of the longitudinal ciliated band in the young 

 stages, and the presence of a dorsal pore. The Echinoderm- 

 larva is not, however, provided with eye-spots, nor has it the 

 posterior, or transverse, ciliated band of Tornaria. 



Eecent studies on the development of Echinoderms ^ have 

 made it probable that the five body-cavities of Balanoglossus are 

 represented in the larvae of those animals ; and this materially 

 strengthens the probability of the view that the respective adults 

 are also allied.^ It may be added that the relationship which 

 appears to be indicated is between Balanoglossus and the bilateral 

 ancestors from which the radially-symmetrical Echinoderms are 

 probably descended. 



In comparing the Enteropneusta with the Pterobranchia, the 

 disproportionate size of the trunk of Balanoglossus may perhaps 

 be explained by assuming that the region of the third body- 

 cavities has been enlarged since Balanoglossus branched off from 

 the ancestral stock." The approximation of the anus to the 

 mouth in Pterobranchia is perhaps the result of their tubicolous 

 habits.^ In the position of the central nervous system in the 

 skin of the collar, GeioJicdodiscus appears to be more primitive 



1 See JIacBride, Quart. J. Micr. Sei. xl. 1898, p. 589 ; xliii. 1900, p. 351. 



- Bury, Quart. J. Micr. Sei. xxix. 1889, p. 409 ; xxxviii. 1896, p. 125 ; MacBride, 

 ibid, xxxviii. p. 395 ; Masterman, Tr. R. Soc. Edinb. xl. Ft. ii. No. 19, 1902, p. 403. 



' This view was definitely formulated by Metsohnikoff in 1881 (Zool. Anz. iv. 

 1881, pp. 139, 153). 



■» Of. Morgan, J. Morpliol. v. 1891, p. 445; ix. 1894, pp. 64-66. 



5 Cf. Lang, Jena. Zeitschr. xxv. 1891, p. 1. 



