;i6 THE ANIMAL KINGDOM. 



Beroidae, in the Hydrozoan Protohydra, Microhydra, Limnocodium and some 

 Medusae. The pores leading to the gastric cavity, or its parts, are a peculiar feature ; 

 but aboral pores are found in Ctenophora pores on the tentacles, peristome, and 

 body-wall and base in some Anthozoa Zoantharia ; pores or subumbrellar papillae on 

 the circumferential canal of some Hydrozoan Leptomedusae. These structures are, 

 however, by no means homologous, and the pores of Sponges have a function found 

 in no other Metazoa. Their great development renders possible the irregular and 

 continuous growth of a Poriferan, and is no doubt correlated with a unique phenome- 

 non, the fixation of the animal by the gastrula-mouth, afterwards obliterated, which 

 has been noted in two thoroughly established instances. Add the collared endoderm 

 cells and the absence of cnidoblasts and adhesive cells ', and the sum of peculiarities 

 certainly gives the Porifera an isolated position among Coelenterata, but whether 

 sufficient to erect them into a group of coordinate value is doubtful. They are re- 

 tained here in this division of Metazoa on account of the resemblance in their 

 fundamental structure (ecto-, endo-derm and mesoglaea) to what is typical of other 

 Coelenterates in general. 



For a remarkable ambulatory colonial Coelenterate, Polypodium ambulans, with 

 non-tentaculate zooids, devoid of mesenteries, but resembling histologically an 

 Actinian, see Korotneff, Z. A. ix. 1886. 



CLASS CTENOPHORA. 



Non-colonial, free-swimming and pelagic Coelenterata, globular, cylin- 

 drical, rarely band-like in shape. The mouth is at one pole of the principal 

 axis, a sensory organ and otolithic mass with two excretory apertures at the 

 other. Eight rows of ciliary or ctenophoral plates radiate meridionally from 

 the sensory organ. There is, as a rule, a pair of tentacles, retractile into 

 pouches, and provided with adhesive cells. Hermaphrodite. Exclusively 

 marine. 



An axis passing through the otolithic mass and the mouth is the prin- 

 cipal axis of the body. It has two poles, an oral and aboral. Two vertical 

 planes at right angles to each other pass through this axis. One is parallel 

 to the longer diameter of the stomach, and may be termed the stomachal 

 or lateral plane : the other is parallel to the longer diameter of the funnel, 

 and may be termed the funnel plane, median or sagittal plane. The funnel 

 plane divides the body into a left and right half, the stomachal into an 



1 How far weight can be laid on this negative character is doubtful. Thread-cells are found in 

 Infusoria among Protozoa ; in Aeolidia {Gastropoda) ; and structures which are generally compared 

 with them in Turbellaria. The late Professor Balfour thought it possible that the layers, i. e. 

 ' ectoderm, endoderm and mesoblast did not correspond with the similarly named layers in the 

 Coelenterata and other Metazoa.' He was influenced in his views by the peculiar character of the 

 amphiblastula larva and by current opinions on the function of the collared cells that they were 

 respiratory, not digestive. There are, however, different forms of Sponge larvae ; and authorities are 

 even as yet by no means agreed as to the function of the collared cells. 



