276 Introduction to Animal Morphology. 



chiolin ; when calcareous it is rich in phosphates, and con- 

 sists of a periostracum, a layer of vertical prisms bent a little 

 at their inner ends, and an irregular granular layer within ; in 

 shape it may be of united dorsal and ventral valves, globular, 

 acicular or spiral, sinistral in Spinalis, Limacina, Heterofusus, 

 dextral in Cheletropus. There is a retractor muscle for the 

 drawing in of the body. The shell-less forms have an inte- 

 gument of cellular connective tissue, with longitudinal (ex- 

 ternal) and circular (internal) muscular fibres,* containing 

 calcareous grains, pigment cells, f and fat-secreting glands 

 under a layer of cylinder epithelium. 



The mouth is anterior, round, or slit-like, with or 

 without lips ; in Tiedemannia it is on a proboscis. There 

 is a radula present (except, perhaps, in Halopsyche 

 and Tiedemannia), whose base is retractile into a 

 sheath ; it is armed with five rows of teeth, an 

 axial series bordered by two intermediate and two 

 lateral rows. In Clione there are bundles of lateral 

 teeth at each side of the mouth, whose bases are im- 

 bedded in muscular tissue. Clione and Pneumoder- 

 mon have eversible sacs, one at each side of the 

 radula, sunk in deep pits, and armed with recurved 

 hooks ; these can be protruded as prehensile organs. 



The straight oesophagus has a crop-like dilatation 

 in Tiedemannia, and ends in the oval or fusiform 

 stomach, which has strong muscular wallsj, some- 

 times with teeth of conchiolin and longitudinal folds 

 (Hyalea). The pylorus is at the centre of the length 

 of the stomach, and the short intestine has first a 



* Either smooth, homogeneous, of fused cells, or branched, stellate 

 fibre-cells. 



t Tiedemannia has stellate cliromatophores, with or Avithout muscular 

 processes. 



X Circular externally, longitudinal internally ; \A'Ca. a connective basis 

 and a ciliated linmg. 



