174 CALIFORNIA ACADEMY OF SCIENCES. [Peoc. 3D Ser. 



length are variable as to the number of somites, while those 

 which have few somites are remarkably constant in this 

 particular. I have never examined a specimen of Polynoe 

 brevisetosa or P. reticulata that did not have 37 somites and 

 18 pairs of elytra; nor have I any specimen of P. squamala 

 with more or less than 27 somites and 12 pairs of elytra 1 . It 

 is very different with Polynoids possessing many somites; 

 these follow the law of all elongated Annelids in having no 

 fixed number of somites. 



In the present species, for example, the number of somites 

 in nine specimens of various sizes ranges from 81 to 86. 

 The number of pairs of elytra in the same nine specimens 

 varies from 41 to 47. But the most astonishing fact in 

 regard to the elytra is the prevailing asymmetry of their 

 arrangement. Out of the nine specimens examined, only 

 three had the same number of elytra on the right and left 

 sides, and, even among these three, two had each two 

 asymmetrical somites, one elytrophorus on the right, the 

 other on the left; so that even here the general symmetry 

 was not real, but the simple result of balancing two asym- 

 metrical somites. A dorsal cirrus is invariably present on 

 the opposite side of the somite or somites bearing an extra 

 elytron (fig. 49). 



I can offer no explanation for this curious asymmetry 2 . 

 Its very frequent, almost universal, occurrence precludes 

 considering it a monstrosity. It is worthy of note, although 

 not an explanation of the anomaly, that the extra elytron 

 always occurs in the posterior region, where the arrange- 

 ment of elytra is very different from that found further 

 cephalad. As far as the thirty-third somite the sequence of 

 the elytra is absolutely the same as in P. brevisetosa, P. re- 



1 The only exception I have noticed is Harmotlioe hirsuta, which has 37 to 40 somites. 



2 The only other instance of such asymmetry among the Polynoidse (or in fact among 

 the Polychaeta generally) that I have found any record of is Lcpidamctria. commensalis 

 Webster (H. E. Webster: "Annelida Chanopoda of the Virginian Coast," Trans. Albany 

 Inst., Vol. IX, pp. 210, 211, 1879). This Polynoid is commensal with Amphitrite ornata; 

 hence its mode of life is precisely like that of P.gigas. It is stated by Webster to have3S-50 

 elytra on a side, which " cannot be enumerated in pairs, since sides of the same seg- 

 ment may bear, one, an elytron, the other a dorsal cirrus. For the first 32 segments the 

 arrangement is uniform. After the thirty-second segment, no two specimens present 

 the same arrangement." 



