complicated body of a true Monstrillid. In the last larval stage the enclosed 

 Monstrillid is very distinctly traced through the thin outer cuticle of the larva, 

 filling up nearly the whole inner cavity, and exhibiting all the definitive appen- 

 dages, as also d'Stinct traces of the generative organs, at least the female 

 ones. At this time the larva is ready to force its way out of the body of the 

 host, and this is apparently accomplished by a burrowing action of the hind 

 acutely produced extremity, which is surrounded with several rows of small 

 hooklike spinules, After the escape of the larva its thin outer cuticle soon 

 burst, and the enclosed Monstrillid is thus allowed to emerge, unfolding its 

 several appendages Its movements are at first lather slow, but very soon, by 

 a single moult, it aquires its full development, moving quickly about, to 

 commence its free pelagic existence 



Gen. 2. Monstrilla, Dana, 1848. 



Syn: ? Thaumaleus, Kroyer (not Giesbrecht). 



Generic Characters. Body as a rule very slender and elongated, especially 

 in female, with the cephalic segment in most cases occupying rather more than 

 half its length and more or less cylindrical in shape; exposed part of trunk 

 gradually narrowed behind, with the lateral parts of the segments rounded off. 

 Tail composed in female of 3, in male of 4 well defined segments, the last 

 one rather small and obtusely truncated behind. 1 Caudal rami sublamellar, 

 each provided in female with 5 or 6 setae, one of which is generally shorter 

 than the others and attached more dorsally; those in male of same appearance 

 as in female, but lacking one of the setae. Eye in most of the species imper- 

 fectly developed. Oral tubule generally fai remote from the frontal part 

 of the head. Antennae more or less elongated, with the joints generally imper- 

 fectly defined in female. Natatory legs with the basal part very massiv, rami 

 comparatively short, buth clothed inside and at the end with long plumose 

 setae; outer ramus somewhat longer than the inner and only provided with 

 2 small spines outside, the one attached to the 1st joint, the other to the end 

 of the last joint. Last pair of legs in female sublamellar and somewhat extended 

 laterally, inner edge more or less expanded, extremity slightly exserted and 

 provided with 3 (in some cases only 2) plumose setae; those in male much 

 reduced in size, knob-like, and tipped with one or two slender setae. 



Remarks. The present genus was established as early as in the year 

 1848 by Dana, and may be legarded as the type not only of the family Mon- 

 strillidae, but also of the whole division here treated of. It is chiefly distin- 



