150 HOMOLOGIES. 



lateral trunks, the blood in each group flowing from behind forward in the dorsal. The much 

 greater differentiation of the branchial region in Balanoglossm necessitates a corresponding com- 

 plexity of the vessels, yet there is a connecting link in the elaborate plexus in the oesophageal 

 region of the Nemertean. 



In regard to generation and development there is also a parallelism. The reproductive 

 elements are developed in sacs in both, and the sexes are often distinct. Certain of the young 

 in each case undergo a kind of metamorphosis, as shown on the one hand by the description 

 already given, and on the other by the interesting observations recently made by E. Metschnikoff 

 on Tomaria} apparently the early condition of Balanoglossus. The occurrence of eye-specks in 

 the anterior region (" proboscis ") in the latter would seem to indicate that the above view of its 

 homologies is correct. 



Having thus examined the relations and homologies of the Nemerteans with their inferiors 

 and apparent equals in the scale, we may next inquire into their affinities with the higher 

 annelids. Here, however, there is room for very diverse opinions, since, so far as known, there 

 are no intermediate forms through which they may be linked on to any higher group. 



Their relationship would rather appear to be with the Leeches than with the Gephgrea or 

 Scoleidce of Prof. Allman's classification, although a considerable gulf intervenes. Thus, in regard 

 to the cutaneous system the cilia are not present in the leeches, though the exudation of the cutis 

 proper is abundant enough. The muscles of the body-wall are less definitely arranged in the 

 latter {e.g. Nephelis), the internal longitudinal bundles for instance being placed in the body- 

 cavity, and separated by regularly arranged vertical fasciculi at the lateral regions. The external 

 coat is composed of circular fibres, within which lie a decussating series. The digestive 

 system opens by a mouth in front of the ganglia, after passing through a nervous collar, and the 

 muscular oesophageal region is distinguished from the more glandular stomachal portion, as in 

 the Nemerteans. There are no cilia in the alimentary chamber, but it is occasionally furnished 

 with csecal processes. The alimentary canal adheres as much to the body- wall in the Leeches as 

 in the Nemerteans, which in this respect differ from the higher Annelids. The dorsal and the 

 two great lateral vessels of the leech are probably homologous with the three vessels of the 

 Enopla, but the ventral is additional. 



In regard to the nervous system, the superior lobes of the Nemertean brain seem to 

 correspond with the supra-oesophageal ganglia of the leech, and the inferior (from which the 

 great lateral trunks arise) with the sub-cesophageal. If in the Enopla the two ganglia were 

 separated, and the lateral nerve-trunks thrown together in the median line of the body, the 

 alimentary canal would become dorsal in position, and would perforce pass through the nervous 

 system to open ventrally, while the lateral vessels would remain in their usual situation. Thus a 

 partial resemblance to the state in the leech would ensue. A much greater amount of branching 

 of course would occur after the concentration of the nervous system. 



The two cephalic sacs and coiled ducts in the Enopla may be the homologues of the 

 segmental organs in the leech. 



1 < Zeitsch. f. w. Zool./ Bd. xx, p. 131, taf. 13, 1870. 



