284 INVERTEBRATE MORPHOLOGY. 



means uncommon, the single reproductive gland producing 

 both ova and spermatozoa and being therefore an ovo-testis. 

 Accessory structures are frequently added to the essential 

 parts of the reproductive apparatus, such, for instance, as al- 

 buminiparous glands, intromittent organs, spermatophore- 

 sacs, etc., so that a relatively complicated arrangement may 

 occur. 



I. CLASS AMPHINEURA. 



The Aniphineura are Mollusca in which the primitive bi- 

 lateral symmetry is fully retained and which seem to approach 

 most nearly to what may be considered the primitive Molluscan 

 condition. All the known members of the group are marine 

 in habitat and are more or less elongated forms in which the 

 elongation of the ventral surface or foot is accompanied by a 

 corresponding elongation of the visceral complex, which ac- 

 cordingly is not elevated at right angles to the long axis of 

 the foot to form a visceral dome. In a general way, therefore, in 

 the form of their body the Amphiueura may be compared to the 

 Platyhelminths, especially to such forms, sometimes flattened, 

 sometimes more or less cylindrical and elongated, as are found 

 among the Poly clad Turbellaria. The mouth and anus are situ- 

 ated at the extremities of the body, and to either side of the anus 

 are situated the one or more pairs of plumelike branchiae and the 

 openings of the single pair of nephridia. The shell may con- 

 sist either of a number of scattered calcareous spicules im- 

 bedded in or projecting from a thick cuticle, or else may take 

 the form of a number of plates arranged in a longitudinal 

 series upon the dorsal surface of the body, and as a rule the 

 mantle-fold is but slightly developed and may be in some forms 

 almost rudimentary. The foot, too, which is so characteristic 

 for the Mollusca, may in some forms be practically un- 

 developed, but in other forms is a broad flat muscular surface, 

 showing no differentiation into special regions such as are 

 found in the higher Mollusca. 



Little need be said here as to the internal organs except to 

 emphasize the fact that both the heart and the nephridia have 

 a perfectly bilateral arrangement. The nervous system is 

 characterized by the absence of a definite aggregation of the 



