56^ THE ANATOMY OF INVERTEBRaTED ANIMALS. 



while the enderon of the integument often becomes the seat 

 of calcareous deposit. The endoskeleton and the dermal exo- 

 skeleton of the Vertehrata are cellular (cartilage, notochord) 

 or fibrous (connective tissue) modifications of the mesoderm, 

 wliich may become calcified (bone, dentine). Recent investi- 

 gations tend to show that the enamel of the teeth is derived 

 from the ectoderm. 



The Alimentary Apjmratiis. — From the simple sac of the 

 Hydra or aproctous. Itirbellarian^ we pass to the tubular ali- 

 mentary tract of the proctuchous Turbellaria, In the Hoti- 

 fera and Polyzoa there is a marked distinction into buccal 

 cavity, pharynx, oesophagus, stomach, and intestines ; while 

 distinct salivary, hepatic, and renal glands, are found in the 

 majority of the higher invertebrates, and, not unfrequently, 

 glands secreting an odorous or colored fluid appear in the 

 region of the termination of the alimentary canal. 



The oral and gastric regions are armed with cuticular 

 teeth in many Invertehrata ; but teeth formed by the calcifi- 

 cation of papillary elevations of the enderon of the lining of 

 the mouth are confined to the Vertehrata ; unless, as seems 

 probable, the teeth of the Eclnnidea have a similar origin. 



The lining membrane of the oral cavity is capable of being 

 everted, as a proboscis, in many Invertebrata. The margins 

 of the mouth may be raised into folds, armed with cuticidar 

 plates. In the Vertehrata, the jaws are such folds, supported 

 by endoskeletal cartilages, belonging to the system of the 

 visceral arches, or by bones developed in and around them ; 

 but, in the Arthro2yoda, what are usually termed jaws are 

 modified limbs. 



The Blood and Clrcidatory Apparatus. — In the Coelen- 

 terata, the somatic cavity, or enterocoele, is in free commu- 

 nication with the digestive cavity, and not unfrequently 

 communicates with the exterior by other apertures. The fluid 

 which it contains represents blood ; it is moved by the con- 

 tractions of the body, and generally by cilia developed on the 

 endodermal lining of the enterocoele. In the Turhellaria, 

 Trematoda, and (Jestoidea, the lacunas of the mesoderm and 

 the interstitial fluid of its tissues are the only representatives 

 of a blood-vascular system. It is probable that these com- 

 municate directly with the terminal ramifications of the water- 

 vascular system. In the Hotifera, a spacious perivisceral 

 cavity separates the mesoderm into two layers, the splanch- 



