164 GENERAL COXSIDERATIOXS. 



favorable material may be seen to terminate proximally in fibres passing into 

 the muscle layer. Distally they attach to the bottom of minute depressions 

 in the cuticle and therefore probably are sensory elements. 



In two specimens the cloacal cavity has been widened greatly, completely 

 exposing the anus and the openings of the shell gland and the more ventral 

 diverticulum. This last named organ has been almost completely everted. 

 These individuals appear in all respects to be normal. 



With the stains employed the nervous system is not sharply differentiated 

 from the surrounding tissue, and accordingly but little has been determined 

 save that relating to the larger ganglia and nerves. As may be seen (Plate 5, 

 fig. 6), the brain holds the usual position and gives rise to the customary nerves 

 distributed to the atrium and the body wall and more posteriorly to the pedal, 

 lateral, and labio-buccal connectives. The pedal cords are considerably enlarged 

 at their anterior ends, while the lateral show scarcely any modification. Pedal 

 commissures and latero-pedal connectives occur at fairly regular intervals through- 

 out the entire length of the animal. The labio-buccal ganglia are located at the 

 sides of the pharynx a short distance behind the level of the salivary glands, 

 and are united by at least one ventral commissure. In the posterior end of 

 the body the relations of ganglia and nerves are not especially clear, but the 

 pedal cords appear to end in small enlargements, united with the termination 

 of the lateral ganglia by one or two slightly enlarged connectives. As usual 

 nerves pass into the hinder ]:)art of the animal from the ends of the lateral ganglia 

 which are united by the usual suprarectal commissure. 



GENERAL CONSIDERATIONS. 



If unanimity of opinion be any criterion whereby we may judge the correct- 

 ness of a theory it must be admitted that we are yet a long way from the solution 

 of the origin of the MoUusca, for scarcely any two investigators hold identically 

 the same views. In their development or in. their adult organization many of 

 the members of the phylum exhibit features \yhich are the close counterpart 

 of others in the flatworms and annelids and it may well be that, generally speak- 

 ing, those students are correct who hold to the idea that all have descended 

 from a common ancestor, though the details of the process are most obscure. 

 Narrowing down the problem to the Solenogastres there are few who dissent 

 from the opinion that they are true molluscs, though it cannot be said their 

 position within the group is definitely estabhshed. However it is becoming 



