16 B Canadian Arctic Expedition, 1913-18 



SPIONIDAE. 

 Spio mimus, n. sp. 



The one specimen of this form secured is about 15 mm. long. It is broadest 

 anteriorly near the ninth somite, in front of which it is pointed and caudad 

 of which it narrows continuously to the posterior end at first gradually and then 

 more strongly. The specimen is complete and consists of forty-five segments 

 or near that number. 



Type specimen. Cat. No. 35, Victoria Memorial Museum, Ottawa. 



The prostomium protrudes anteriorly in a rounded process much as in 

 S. filicornis. The sides are subparallel back to the level of the first parapodia 

 from where they converge caudad, the caudal end of the prostomium being 

 narrowly rounded and lying at the anterior edge of the second setigerous somite. 

 The median ridge of the prostomium rises posteriorly and expands into a rounded 

 elevation on which the eyes are borne. There are two pairs of eyes with slight 

 pigmentation on one side indicating a possible intermediate eye. The posterior 

 eyes are smaller and much nearer to each other than the anteriors. (See PI. Ill, 

 fig. 1). 



Parapodia all of the usual well-developed biramous type. Both notopodium 

 and neuropodium throughout have well-developed postsetal lamellae which in 

 general are low, vertically elongate and evenly rounded. The neuropodial 

 lamella at the posterior end becomes relatively longer, more cirriform. The 

 dorsal lamella is fused at the base with the branchia. It remains at the same 

 general form over most of the body but at the posterior end becomes gradually 

 increased in length and finally much elongate and branchiform. The branchiae 

 are present on all parapodia excepting the last two pairs. The first branchiae 

 are short, the following ones increasing in length and soon attaining the maximum 

 when they ordinarily extend more or less obviously beyond the middorsal 

 line and are proportionately heavy. At the posterior end the branchiae become 

 reduced and in the last few pairs are much exceeded in length and thickness 

 by the dorsal lamella which in the meantime has become cirriform and some- 

 times a little clavate. The last branchiae occur on the third pair of parapodia 

 from the last and are merely slight tubercles. (See PL III, fig. 3). 



Only capillary setae are present in the first sixteen or seventeen pairs of 

 parapodia. Those of the posterior series are broad, bilimbate setae with fine, 

 often curving tips. The anterior setae are finer. The superior dorsals are of 

 the finer capillary type. On the seventeenth parapodia crochets make their 

 appearance in the neuropodial fascicle, at first one and two in number, and then 

 increasing to nine, which seems to be the prevailing number. The crochets 

 are of nearly uniform width over most of their length, the free portion moderately 

 evenly curving. At the distal end of the shaft narrows to a rather slender 

 neck above which it terminates in two teeth of which the apical is much smaller 

 and acute. Distal end completely sheathed, the sheath widening distad and 

 then rounded at the end. Shaft strongly fibrillate. (See PI. Ill, fig. 4). 



The caudal end of the body ends in four papillae of which the ventral are 

 much stouter than the dorsals, reversing the condition, e.g., in S. mesnili (Euspio 

 Mclntosh). (See PL III, fig. 2). 



LOCALITY. Alaska: Collinson point. Station 27o. September 20, 1913. 

 " Pelagic under 5 inches of ice over 1 foot of water. Lagoon at Collinson point." 

 One specimen. 



This species resembles S. filicornis (O. F. Mtiller). It differs in the character 

 of parapodia and branchiae in the posterior region, the great reduction of the 

 branchiae and the elongate, cirriform or subcirriform character of the lamellae, 

 etc. Contrasting conspicuously with the condition in filicornis, in which the 

 branchiae remain long and the lamellae are not thus modified. The crochets 

 first appear farther caudad than in filicornis, etc. 



