570 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



As this blastodermic disk grows, its margins thicken and give 

 rise to two germ-bands (Keimstreifen). These gradually ap- 

 proximate and eventually unite upon the opposite face of the 

 ovum. As the chain of ganglia is the product of the differen- 

 tiation of the epiblast of the germ-bands, it follows that it is 

 formed by the union of two primarily distinct nerve-tracts, 

 which move round from the hgemal to the neural aspect of the 

 body ; and thus the arrangement of the nervous trunks in 

 Malacobdella ' may be regarded as expressive of a condition 

 which is transitory in Clepsine and JEuaxes. 



Many years ago 2 I directed my attention to the fact that 

 " the development of a Mollusk commences on the haemal 

 side and spreads round to the neural side, thus reversing the 

 process in Articulata and Vertebrata ; " and it is very inter- 

 esting, considering the many curious points of approximation 

 between the Annelida and the Mollusca which are now com- 

 ing to light, to observe that certain Annelids present this 

 especially Molluscan peculiarity. 3 As Von Baer long ago 

 pointed out, there is a striking likeness between the foot of a 

 Gasteropod and the suctorial disk of one of the Hirudinea. 

 The so-called jaws of the Leeches (the " teeth " of which, I may 

 observe in passing, are calcified) are curiously similar to an 

 odontophore devoid of cartilages, the representative of the 

 radula being supported on a muscular cushion. 



The statement at p. 215, that " no calcareous skeleton 

 is found in any of the Gephyrea" ceases to be true since the 



1 According to Semper's recently-published statements, Malacobdella is a 

 true Nematoid, and not a Leech. ("Die Verwandtschaftsbeziehungen der 

 gecrliederten Thiere," " Arbeiten aus d. Zoologisch-zootomischen Institut in 

 Wurzburg," Bd. iii., 1876.) The memoir here cited is full of important obser- 

 vations respecting the structure of the nervous system in the Annelida ; the 

 a^araogenetic multiplication of JVais and Cliwtog aster ; and the development of 

 the organs of these Annelids. 



Moreover, the author discusses very fully the relation of the Annelidan with 

 the vertebrate types of organization. 1 do not propose to touch upon this subject 

 in the present Volume; but I may remark that the evidence upon which the 

 identification of the structures termed " Kiemengangwulste " and their products 

 with the branchial apparatus of vertebrate animals is founded, appears to me 

 to be wholly inadequate to bear out the conclusions deduced from it. 



2 " On the Morphology of the Cephalous Mollusca." (" Phil. Trans.," 

 1S52, p. 45 and note.) 



3 The mode of development of the central nervous system in Euaxes and 

 Clepsine offers many points of interest. Not the least important of them is the 

 obvious similarity (to which attention has already been directed by Semper) 

 between the germ-bands of Clepsine when they have united throughout the 

 greater part of their length, but surround the blastopore behind, and the Am- 

 phibian embryo with its dorsal ridges, which have exactly similar relations. 

 (See, for example, Fig. 40, in Plate III. of GOtte's work, " Die Entwicke'lungs- 

 geschichte der Unke.") 



