772 Affinities of Annelids to Vertebrates. [August, 
and by loss of the connections of the lateral pouches with 
one another and with the digestive tract, thus forming the seg- . 
mented body-cavity and the segmental organs. The vertebrate 
was formed by a more complete union of the lateral parts of the 
nerve-ring between the separated mouth and anus, together with 
disappearance of its two ends, by the persistence of some ante- 
rior pouches to form branchial clefts, by the loss of external 
openings for others which remained connected with one another 
and with the digestive tract at one point to form part of the body- “ 
cavity and the urogenital ducts, and finally by the retention of 
only the outer openings of the last pair of pouches which form 
part of the body-cavity and the abdominal pores. Mouth and 
anus are thus homologous in the two groups, and primitively 
neural in position; the mouth in the vertebrate acquired its pres- 
ent ventral or hæmal position by advancing around the anterior 
end of the body. 
Fig. 7. 
Fig. 6. 
Fic. 6.—Diagram of an annelid according to Sedgwick. M, mouth; 4, anus. 
Fic. 7.—Diagram of a vertebrate according to Sedgwick. JZ, mouth; A, anus, with 
spinal cord between. 
A still more remote separation of the annelids from the verte- 
brates is required by the comparisons instituted by Goette from a 
study chiefly of the development of planarians and annelids ; the 
fundamental ground-plan of the two groups is, he thinks, essen- 
tially different, as shown by the entirely diverse ways in which 
the gastrula is modified in the two to form the adult. In the 
__ annelid the blastopore closes along a line which corresponds to 
_ the longitudinal axis of the adult, the mouth being formed from 
_ its anterior part and the anus near its posterior end; the nervous 
_ cord is formed along this line of union of the lips of the blas- 
topore. Thus the ventral surface of the adult represents the 
