66 Evolution and Distribution of Fishes 



branchial pouches as homologues. The external ear-orifices 

 remain open in both of the above groups, and so this condi- 

 tion forms another point of contact with nemerteans. But 

 the positions of the orifices become gradually shifted till 

 ultimately they open on the latero-dorsal or dorsal surface, 

 (d) Epidermal haiis. The only other external char- 

 acter that deserves notice here is the hair. This seems to 

 be one of the most primitive, as it is the most persistent 

 epidermal sense modification of animals. For the "bors- 

 tcn," "geiselhaare," "haftpapillen," and "hacken" of the 

 rhabdocoels ( J7 : 2012-2030) are all varieties of tactile epi- 

 dermal cells. Such persist among the nemerteans as sens- 

 ory hairs, that Montgomery as well as other authors fre- 

 quently refer to. In the metanemerteans he describes them 

 on the posterior part of the body, on anterior and posterior 

 parts alike, or in somewhat diffuse manner {38: i). Re- 

 garding fishes Bridge says {36'. 383) "the most remarkable 

 and certainly the most characteristic of the sense-organs of 

 Cyclostomes and Fishes are bud-like groups of epidermic 

 cells in relation with the ends of sensory nerve fibres. Each 

 consists of a central core of sensory cells, provided with 

 terminal cuticular sensory hairs, and surrounded by a zone 

 of supporting and mucus-secreting cells which leave the 

 hairs exposed at the apex of the bud." He then describes 

 the two varieties known as "end-buds" and "nerve-emin- 

 ences. "^ 



V. The mouth. 



In all freshwater and land nemerteans the mouth is a 

 small subterminal orifice that opens into a pharynx or 

 buccal cavity of varying size. This is a common cavity 

 that shows on its inner ventral aspect the small opening of 

 the gullet, and behind this a more or less circular ridge 

 that is the attaching edge of the proboscis to the proboscis- 

 sheath. This ridge we will call, for a reason that will 

 appear presently, the velum. It bounds the anterior part 

 or orifice of the proboscis, and so demarcates also the front 

 edge of the proboscis-sheath (Fig. 2m. p.) from the 

 buccal cavity. Glandular cells are embedded in its surround- 

 ing tissue, and evidently pour their secretion into its cavity. 



