344 



ZOOLOGY 



SECT. 



Structure of Body-wall. Beneath the cuticle is an epi- 

 dermis, consisting of a single layer of flattened polygonal cells, 

 firmly united together by their edges. Beneath this there 

 is usually, but not always, a layer of muscle, which is 



arranged in two strata an 

 external composed of circular, 

 and an internal of longitu- 

 dinal fibres. There is an ex- 

 tensive ccelome lined in some 

 forms (Phylactolsemata) by a 

 definite coelomic epithelium, 

 in part ciliated ; while in 

 others there is no such de- 

 finite epithelium, but its place 

 is taken by thin parietal and 

 visceral layers of an irregular 

 cellular tissue the paren- 

 chyma. Crossing the ccelome 

 are strands, in some instances 

 very numerous, of- spindle- 

 shaped cells. In some cases 



FIG. 287. Anterior portion of the body of -, , 



Lophopus, from the right side. an. anus ; tWO mesenteriC bands SUS- 



ep. epistome ; a. ganglion ; o. mouth ; pr. in- npT1 ^I fl^p alirnpnfarv rarial 

 testine ; st. rrsophagus ; t. tentacles, cut off ' alimentary Canai- 



near the base. (From Lang's Comparative an anterior attached near the 



Anatomy. Alter Allman.) 



mouth and a posterior passing 



from the caecum to the aboral end of the zocecium ; in most 

 cases the latter, to which the special name oifuniculus is given, is 

 alone present. 



The alimentary canal has in all species the parts that have 

 been already described in the case of Bugula. In some of the 

 Cheilostomata it is stated that the cells of the oesophagus bear 

 numerous striated muscle-fibre processes. In some Ctenosto- 

 mata there is in addition a thick-walled chamber the gizzard 

 with chitinous teeth, between the oesophagus and stomach. 



The nervous system consists of a single, sometimes bilobed, 

 ganglion (Fig. 285, gang., and Fig. 287, ga.) placed between the 

 mouth and the anal aperture, and of nerves passing from it 

 to the various parts. There are never any organs of special 

 sense, unless the epistome of the Phylactolsemata be of that 

 nature. 



Nephridia are not known with certainty to exist in any of 

 the Ectoprocta. In some there is a pore through which water 

 enters the body-cavity, or a ciliated intertentacular tube opening 

 at the base of the tentacles. Excretion appears to be performed 

 by certain cells of the funicular tissue and of the parenchyma 

 or coelomic epithelium. These become loaded with the products 

 ot excretion, and are set free as leucocytes in the ccvlome, whence 



