6 LEODICID^ OF THE WEST INDIAN REGION. 



prostomium, or, as in Leodice, lying near the bases of the tentacles, and a sensory 

 or "nuchal" organ in the form of a shallow pit lies on either side near the posterior 

 prostomial border, and usually is covered by the anterior margin of the peristo- 

 mium. Practically all of the details of prostomial structure mentioned (except 

 number of tentacles) vary in the living animal with the degree of expansion, and 

 in preserved material with the excellence of the preservation. 



3. Peristomial structure: The peristomium or first somite surrounds the 

 mouth and its anterior margin may, if the animal is contracted, partly cover over 

 the posterior portion of the prostomium. It is usually longer than the second 

 somite, its size being a feature of diagnostic value. It is closely connected to the 

 second somite, and there may be more or less of a fusion of the two, especially 

 on the dorsal surface. This has led to the description of some species as having a 

 biannulate first somite, or in other cases to the statement that only one somite 

 is without parapodia. So far as my observations go, the two first somites are with- 

 out parapodia, but in some cases they are so nearly fused as to look like one. 

 In Lumbrinereis the ventral anterior border of the second somite is prolonged to 

 form the posterior margin of the mouth. The second somite may bear nuchal 

 cirri, whose presence or absence (and if present, their form and size) is of diagnos- 

 tic value. In Omiphis (plate 7, figure 2) it looks as if the nuchal cirri were carried 

 on the first somite. I think that it indicates a fusion of the first two somites. The 

 question might perhaps be settled by a study of the nerve distribution, which I 

 think has not been done for this genus. Pruvot (1885) found that the innervation 

 in Leodice demonstrates that the ring carrying the nuchal cirrus is a true somite, 

 and that the innervation of the nuchal cirrus indicates that it is homologous with 

 the dorsal parapodia! cirrus. 



4. The parapodia: These are of the type called "sesquiramous" by Pruvot 

 and Rachovitza (1895, p. 346), since the hsemal lobe has entirely disappeared, 

 its position being indicated by a tuft of very small seta-like aciculse which extend 

 into the base of the dorsal cirrus. The parapodium has a setal lobe with one or 

 more aciculae, to which is usually added toward the posterior end of the body a 

 large acicula-like structure extending into the ventral portion of the parapodium 

 and protruding to the surface just dorsal to the ventral cirrus. This has been called 

 a ventral acicula, but Pruvot and Rachovitza assert (1895, p. 419) that to call it 

 an acicula assumes that the parapodium has a third (ventral) lobe, which does not 

 occur in any annelid, and that it is reaUy the ventralmost seta, which it usually 

 resembles in having a hood, though in other respects it is like an acicula. They 

 propose to call this an "acicular seta," and this nomenclature has been followed 

 by Fauvel (1914 and 1917). I am unable to agree with this interpretation, since 

 this structure does not arise with the seta bundle but with the other aciculse. It 

 seems to me to be merely an acicula which has come to the surface ventrally to 

 the others. It is, to be sure, hooded, but the ordinary hooded seta is compound, so 

 that the resemblance is not close. The form of the setal lobe and the character 

 of its lips are of diagnostic value, as are also the forms of the dorsal and ventral 

 cirri, though these are not present in all genera. 



