TRANSACTIONS OF SECTION D. 1097 
the body-wall in the region thickens, and an actual budding region is here formed. 
This new region elongates and presents a solid appearance. The alimentary canal 
erows in this region, but the newly-formed portion is at first unpigmented, and 
may still be detected at a much later period by its lighter colour ; its lumen remains, 
however, all the time, and a continuous line of fsecal matter may be observed. 
This budding region divides into two portions. The anterior portion develops 
numerous sete, and gives rise to an indefinite number of segments which form the 
tail of the old worm: the posterior portion develops four pairs of ventral sete. 
This development taking place from before backwards, and subsequently at its an- 
terior region, the characteristic proboscis is developed, and the two individuals 
separate. The budding region usually forms between the 25th and 26th segments, 
so that the 26th segment of the parent worm becomes the 5th segment of the 
posterior daughter worm, the four anterior segments of this worm never presenting 
dorsal sete. This condition, moreover, obtains in all the individuals, whether 
sexual or otherwise, of Nais proboscidea, i.e., there are four anterior modified 
(cephalized) segments, The number of such segments varies in different species of 
the group. 
8. Demonstration of a new Meneron. By Professor D’Arcy W. THompson. 
9. On the Blastopore and Mesoblast of Sabella. 
By Professor D'Arcy W. Trompson. 
10. On the Annelids of the Genus Dero. By E. C. Bousrretp. 
The annelids belonging to the genus Dero are allied by most of their external 
characters to the Naides, but are distinguished from them— 
1. By the absence of eyes. 
2. By the absence of corpuscles from the perivisceral fluid. 
3. By the termination of the body ina wide membranous expansion bearing 
four branchial processes. 
This expansion, or branchial area, is essentially a prolongation and opening out 
of the posterior part of the intestine, and is covered on its non-ciliated surface by 
the general integument of the body. It is highly contractile, this property being 
due to the presence of numerous stellate muscle-cells between the respiratory and 
epidermal walls. Between these walls also run the blood-vessels, the arrangement 
of which is in this part of their course much modified from the type which 
characterises the genus Nais. The abdominal vessel runs along the middle of the 
branchial area and divides at its termination into two branches, which run round 
the area, giving off looped branches to the branchial processes (one to each) ard 
also branches which cross the area obliquely. The branches of each side unite in 
a common trunk, and the two trunks together furm the dorsal vessel. The bran- 
chial processes are elevations of the ciliated surface of the area, and as already 
stated contain a looped blood-vessel, this being surrounded by a hollow cone of 
muscular elements, by means of which the processes are lengthened or shortened 
at the will of the animal. 
Very various accounts are extant with respect to the number of the processes, 
but in no case do more than four arise from the floor of the area, and sometimes: 
two supplementary ones of much smaller size on the margin of the latter, where it 
joins the dorsal surface of the body. Wherever a greater number has been de- 
scribed than three pairs, it is due to the fact that the expanded edges of the 
branchial area, seen in profile, or perpendicularly, have been mistaken for addi- 
tional ones, and to arrive at a correct estimate it is absolutely necessary to examine 
the animal without any pressure whatever. The form of the processes varies in 
different species. It is cylindrical in D. Perrier’ and palpiyera ; flattened-cylindrical 
in Jatissima; foliate, with rounded apices, in phelippinensis, limosa, and obtusa ; 
