1897-98.] Dr Masterman on Archimeric Segmentation. 279 
We can, however, find evidence of its existence, and more espe- 
cially of its leading feature, that of archimeric segmentation and 
its attendant phenomena, in the anatomy and ontogeny of present- 
day species. 
(B.) We are thus led to the general characters of the Archi- 
cceJomata as follows : — 
Morphological. 
1 . Body divided into three more or less clearly defined segments, 
one pre-oral and two post-oral. 
2. Mesoderm forming coelomic cavities corresponding to the 
archimeric segments, and primitively opening to the exterior by 
ciliated ducts. The first or protocoele is essentially muscular, sensory, 
and locomotive, and is present throughout life or only in early 
stages, — the second or mesocoele connected with food ingestion and 
primarily produced into a series of post-oral tentacles which may 
(Brachiostoma) or may not persist ( Balanoglossus , Chcetognatha), 
and may, upon atrophy of the protomere, assume the locomotory 
function ( Echinodermata ), — the third or metacoele connected 
primarily with the gonads and the vegetative functions. A chon- 
droid mesoblastic skeleton is of very general occurrence. 
3. Nervous system mostly in continuity with the ectoderm, and 
consisting of a protomeric ganglion (brain), a mesomeric ganglion 
(sub-oesophageal), and a post -oral, mesomeric, ring connecting 
them. A diffuse plexus in parts, and more or less prominent 
metameric bands, which are unsegmented. 
4. Vascular system, if present, very simple, consisting of 
sinuses between the coelomic epithelia (dorsal and ventral trunks 
and a post-oral ring-sinus), and a central archimeric heart or sub- 
neural sinus, in close connection with which there may or may not 
persist the primitive archimeric excretory organ, or sub-neural 
gland. 
5. No indication of a true metameric segmentation. 
6. Gonads arise from wall of metacoeles, and in some cases have 
idiodinic ducts (probably derived from metacoelic nephridia), in 
others, the metacoelic nephridia function as gonaducts. 
