38 COMPARATIVE ANATOMY. 



mals excepting Cephalopoda, Crustacea, terrestrial arthropod 

 Articulata, and Pisces, excepting Amphioxus. 



Spermatic filament. (See page 103.) 



Among accessories to motion are erectile and elastic tissue. 



Erectile Tissue. A periodic turgescence and collapse of 

 certain tissues (erectile) by alternate excess and comparative 

 absence of blood, may be considered a source of motion, as 

 witnessed in the power of accommodation of parts of eye in 

 fishes, through choroid body ; and probably in the dilatation 

 and contraction of the pupil of the eye of Mammalia, and in 

 the erection of the penis of animals of the same class. 



Elastic Tissue. This restores parts to their original posi- 

 tion from which they have been moved by muscular action, 

 as by nuchal ligament of vertebrates ; or maintains patulency 

 of vessels, as in arteries; or aids to sustain the weight of vary- 

 ing bulks of tissues, as in the abdominal muscles of animals 

 with large paunches, as in Elephas (elephant), and Rhinoceros. 



The pedicle of Vorticella, and lassocells of Hydra also 

 probably possess the property of elasticity. 



VI. 



TEETH. 



TEETH are those sclerous appendages to the pre-assimilative 

 portion of the alimentary canal subservient to the prehension 

 or trituration of food. 



They may be calcareous, siliceous, corneous, chitinous, 

 or dentinal. In Invertebrata, the basis of tooth structure is 

 commonly same as basis of skeleton, and is either developed 

 in the same manner, as in the majority of Articulata, or in a 

 special sac, as in tooth-bearing Mollusca and Nemertes. In 

 Vertebrata, the dentinal sac is constantly present, is not 

 seen in the skeleton, and arises from a formative bulb. 



False Teeth. ' Hinge teeth* of bivalve molluscs, as Unio 



