414 CTENOPHORA 



drive the animal slowly through the water with the oral cone 

 forwards. In some Ctenophores the costse are phosphorescent.": 



Tentacles. — In all the Ctenophora, except the Beroidae and 

 the adult stages of Lobata and Cestoidea, there is a single pair 

 of tentacles. They are attached to the base of a blind funnel- 

 shaped pit which opens to the exterior near the equator of the 

 animal's body. The pits are on opposite sides of the body, 

 and the plane which passes through them both vertically divides 

 the body into approximately equal parts. It is called the 

 " tentacular " or " transverse " plane (Fig. 180). The plane at 

 right angles to this, which also passes through the mouth and 

 statocyst, is called the " sagittal " plane. 



The tentacles are solid, and in the Cydippidae, of con- 

 siderable length. During life they are usually extended, and 

 trail behind the animal as it progresses through the water. , 

 But they are extremely contractile, and when the animal is 

 alarmed are suddenly withdrawn into the shelter of the tentacular 

 pits. Each tentacle usually bears a row of short pinnae. The 

 surfaces of the tentacles and of their pinnae are crowded with 

 remarkable cells which carry little globules of an adhesive 

 secretion, and are called the glue-cells or " colloblasts." These 

 cells stick to any foreign body they touch, and may be drawn 

 out some distance from the tentacle, but they remain attached 

 to it by a long spiral thread which unwinds as the cell is pulled 

 out. Although the colloblasts have the function of catching 

 prey similar to that of the nematocysts of Coelenterata, they 

 are true animal cells and are not therefore homologous with 

 nematocysts, which are the cell products of the cnidoblasts.^ 



The Lobata and Cestoidea pass through a stage in develop- 

 ment called the Cydippiform or Mertensia stage, when they 

 possess a single pair of long tentacles similar to those described 

 above. In the adult condition, however, these tentacles are 

 absent, and their functions are performed by numerous small 

 accessory tentacles or tentilla arranged in rows on definite lines 

 along the body-wall. 



Sense-organ. — At the aboral pole of the Ctenophore there 

 is a hard grapulated calcareous body, the " statolith." This is 



1 A. W. Peters, Journ. ExiKr. Zool. ii. (1) 1905, p. 103. 



^ Cnidoblasts are stated by Chun to occur on the tentacles of Suchlora ; and 

 batteries of " nettle cells " by Abbott on the tentacles of Coeloplana. 



