lo ZOOTOMY. 



body above and at the sides, while below it is separated by 

 a deep furrow (Fig. 4, gr). 



22. The mouth (Fig. 4, m), situated at the bottom of 

 the oral funnel, and of a somewhat crescentic form. 



23. The tongue (Fig. 4, /), the end of which forms a 

 well-marked protuberance bounding the mouth below, and 

 produced on either side into an upwardly directed, wing-like 

 process (t). 



24. The yellow horny teeth, which beset the inner 

 surface of the oral funnel and the tip of the tongue : one 

 immediately above the opening of the mouth is large and 

 bifid, and is distinguished as the maxillary tooth ; those 

 on the tongue are the lingual teeth ; and a transverse 

 crescentic ridge, below the tongue, and produced into 

 several cusps, is known as the mandibular tooth. 



In P. marinus the two cusps of the maxillary tooth are close together, 

 and the lingual teeth consist of two pairs of lunate denticulate ridges, 

 the two ventral united with one another in the middle line. In P. 

 fluviatilis the cusps of the maxillary tooth are somewhat widely sepa- 

 rated, the dorsal lingual teeth are absent, and the ventral are united to 

 form a strong transverse ridge, with a promiuent median cusp. 



25. The nostril, a single median aperture on the upper 

 surface of the head, a short distance from its anterior end. 



26. The eyes, situated at the sides of the head, a little 

 posterior to the nostril ; they are devoid of eyelids, but 

 covered with transparent integument. 



27. The external branchial apertures (Figs. 4 and 6, 

 e.a), a row of seven small slits on each side of the head, 

 the first a little behind the eye. 



28. The urinogenital papilla (Fig. 5, u.g.p), a small 

 elevation, situated in the median ventral line, at about a 

 quarter of the length of the body from the hinder end. It 

 is pierced at its summit by a small opening — the urino- 



