STRUCTURE 415 



supported by four tufts of fused cilia, and is usually covered by 

 a dome of delicate protoplasmic texture, which is believed to be 

 formed by a fusion of cilia. The dome enclosing the statolith 

 is called the " statocyst." 



Supporting the statocyst there is a circular or oval area of 

 ciliated epithelium which is usually supposed, but on insufficient 

 evidence, to be specially sensory in function. Extending from 

 this area in the sagittal plane there are two strips of ciliated 

 epithelium called the " polar fields." 



The aboral sense-organ of the Ctenophora is one of the most 

 characteristic organs of the Phylum. The aboral pole of the 

 Medusae of Coelenterata is usually devoid of any special modi- 

 fication of the ectoderm of the bell, and in the Tiarid genus 

 Stomatoca the little tassel at the aboral pole of the Medusa 

 cannot in any sense be regarded as a homologue of the sense- 

 organ of the Ctenophore. If the aboral sense-organ of the 

 Ctenophora can be compared with that of any other group of 

 animals, it would be with the statocyst of many of the 

 Turbellaria, such as that of Convoluta, but it is far more 

 satisfactory to regard it as an organ peculiar to the Ctenophora 

 and as having no true relationship with any sense-organ found 

 in other animals. 



Alimentary Canal. — The mouth of the Cydippiform 

 Ctenophores opens into a sac-like chamber called the " stomo- 

 daeum," flattened in the sagittal plane and stretching from the 

 oral pole as far as the centre of the body. The stomodaeum 

 passes into a chamber flattened in the transverse plane called the 

 " infundibulum." From the infundibulum a narrow tube passes 

 in the direction of the aboral pole called the " intestine," and 

 from the extremity of this four short tubes pass to the sides 

 of the polar fields at the place where these fields join the sensory 

 area. Two, or, in some cases, all four of these tubes open to the 

 exterior ; but they do not appear to serve the purpose of ejecting 

 the undigested portions of the food, which usually pass to the 

 exterior by the mouth as in Coelenterata and Turbellaria. 



From the lateral extremities of the infundibulum four pairs 

 of tubes pass to the equatorial region of the body, where each one 

 joins a longitudinal vessel which runs immediately beneath the 

 epithelium supporting the ribs. These are called the longitudinal 

 or " sub-costal " canals. From the infundibulum there also 



