424 ZOOLOGY 
contain but few species; the fourth group, however—the 
Vertebrata—has, from the size of its numerous members and 
the great variety of form they present, and above all from the 
fact that it culminates in the genus Homo, acquired a great 
importance, and it was formerly customary to oppose this 
eroup of Vertebrata to the remainder of the animal kingdom, 
which were classed together under the general heading Inverte- 
brata. This division of the animal world was to a great extent 
supported by the fact that much more was known about the 
anatomy and developement of the comparatively conspicuous 
Vertebrata than about the much less accessible and often 
minute Invertebrata, but in late years our knowledge has been 
much increased in the latter direction, and it is now recognised 
that the group Vertebrata is, from a strictly morphological 
standpoint, of no greater interest than any of the other large 
eroups which compose the animal kingdom, 
Ciass I. Hemichordata. 
CHARACTERISTICS.— The anterior end of the body forms an elon- 
gated proboscis, at the base of which the mouth opens. The 
region surrounding and immediately behind the mouth ws 
termed the “collar,” and this is succeeded by a trunk con- 
taining the generative organs. A series of pores or gill- 
slits are generally present. A diverticulum of the pharyna 
projects forward into the proboscis-stalk and forms the noto- 
chord, The sexes are separate. 
The Hemichordata are a group recently constituted by 
Bateson to contain the genus Balanoglossus. Harmer’s investi- 
gations on the structure of Cephalodiscus have shown that that 
genus must be assigned a place near balanoglossus, and still 
more recently Fowler’s researches have shown that Rhabdo- 
pleura rnust also be included in the group. The two last- 
named forms were previously placed in the neighbourhood of 
the Polyzoa; at present we know nothing of their embryology. 
The group now comprises the three sub-classes (1.) Entero- 
pneusta, (ii.) Cephalodiscida, (111.) Rhabdopleurida, each repre- 
sented by a single genus. 
