418 DR. A. FARRE ON THE STRUCTURE OF THE CILIOBRACHIATE POLYPI. 
nature of the external covering, and other points of resemblance between the two 
classes, are easily traced. 
But between Ciliobrachiata and Rotifera the affinities are still nearer. Taking 
Hydatina as the representative of the latter class, the cilia (which, however, differ in 
form) are placed on short lobes instead of arms. The pharynx is very short, and 
leads at once to the manducatory organ, which guards the entrance to the stomach, 
as in Bowerbanhia. The muscular apparatus for altering the form of the body is 
identical in the two classes, and in the general character of the body and position of 
the contained parts there is a very close resemblance. They vary, however, in their 
inode of reproduction, position of the anus, and other points. 
The Ciliobrachiate polypes being thus separated from the rest of their associates 
by characters well defined and easily recognized, there yet remain two other types, 
which maybe represented by Hydra and Actinia, the Hydriform and the Actiniform , 
or Zoanthiform polypes. 
Of these two, the Hydriform polypes, whilst they are the furthest removed from 
Ciliobrachiata in degree of organization, are nevertheless those which have been most 
frequently confounded with them. For it would appear that in the lower type the 
superficial characters of the higher are sketched, as it were, in outline ; so that whilst 
they are found to differ materially in intimate structure, there yet remains a sufficient 
resemblance in external configuration to have caused them to be confounded together. 
The Hydriform polypes maybe recognized by the granular structure of the body, by 
the entire absence of a stomach distinct from the parietes, by the single external open- 
ing to the cavity, and the absence of cilia from the tentacula. 
The granular parenchyma of the body having a contractile power in every part, the 
alterations in its form and dimensions are effected without the necessity for a distinct 
muscular apparatus. No folding of the body takes place when the animals withdraw 
into their cells, where they are still left more or less exposed from the absence of a 
distinct operculum. The food is received at once into the main cavity of the body 
which constitutes the stomach, there to be acted upon by the granular parietes; and 
whilst the egesta escape by the same orifice by which they were taken in, as in Hydra , 
the nutrient particles have been traced to the tentacula in the free animals ; and in 
the compound ones as flowing in a stream through the tubular fleshy medium which 
communicates with the stomach, and by which all the animals are united. 
As in the higher type the tentacular cilia appear to be concerned both in nutrition 
and respiration, so their absence in the present case must be viewed with reference to 
both these points. With regard to the greater choice of food afforded by the action 
of cilia so placed, this appears to be less necessary where the body is of such a struc- 
ture as to be capable of accommodating itself to prey of a greater variety of size; 
whilst, for purposes of respiration, the exposure of the entire surface of the body to the 
water, which has free access to the cell, may be sufficient, without the necessity for 
that constant renewal of it that a more complicated organization appears to require. 
