172 THE INVERTEBRATA 



eight meridional rows of comb plates formed of strong cilia borne 

 upon modified ectodermal cells. The general surface of the body is 

 not ciliated. 



On opposite sides of the body are two tentacles set in pouches. The 

 tentacles have muscular bases and are capable of being protruded 

 from the pouches or withdrawn again. They are usually about half as 

 long again as the body when fully extended. The tentacles are armed 

 with cells of a special type called "lasso cells" or coUoblasts, which 

 take the place of nematocysts. Each colloblast consists of a sticky 

 head having at its base a spiral thread wound round a stiff central 

 filament. The tentacles are used for catching the prey which is 

 entangled by the sticky heads of the colloblasts. 



The mouth leads through a stomodaeum lined with ectoderm into 

 a space, the infundibulum, lined with endoderm. From the in- 

 fundibulum four canals radiate outwards ; each of these divides into 

 two and then runs under the comb plates as the subcostal canals. Two 

 more canals lead out from the infundibulum and run directly without 

 branching to the base of the tentacles. There are also two para- 

 gastric canals running alongside the stomodaeum. 



At the opposite pole to the mouth, the aboral pole, is the elaborate 

 sense organ formed of small round calcareous bodies united into a 

 morula. This morula is supported on four pillars of fused cilia and is 

 covered by a roof also formed of fused cilia. Ciliated furrows lead 

 out from the sense organ to the comb plates and are believed to assist 

 in carrying stimuli to the comb plates from the sense organ. 



The comb plates are the locomotor organs. When at rest the tip of 

 a plate is directed towards the oral pole. In movement a rapid beat 

 of the plate is directed aborally and the cilia then return slowly to 

 rest. The ctenophore therefore moves slowly through the water with 

 the oral end in front. Each plate of the comb beats in succession, the 

 first plate to beat being the one at the aboral end and the remainder 

 following in succession. This type of beating, which is common in 

 ciliary movement, is termed " metachronal" (see p. 13). It gives the 

 appearance of waves travelling down the comb from the aboral to the 

 oral pole. Ordinarily all the eight rows of plates beat in unison, but 

 interference with the aboral sense organ destroys this unison. 



The main substance of the ctenophore, which fills the space 

 between the ectoderm and the endoderm, is a gelatinous material in 

 which are found strands of muscle. Immediately beneath the ecto- 

 derm lies a subcuticular layer of muscle and nerve fibres which, in 

 appearance, closely resembles the arrangement found in the Turbel- 

 laria. It is important to note that the whole musculature of the 

 Ctenophora is derived from the mesenchyme. There are no mus- 

 culo-epithelial cells. 



