OCR Text |
Show 1895.] HYDBACHNID POUND IN COENWALL. 177 not occur on the larger plates. The larger plates are arranged in three irregular longitudinal rows; the same plate rarely exactly agrees in form or size in two individuals or on opposite sides of the body, and seldom, in form, even on the two sides of the same plate ; but the arrangement may be said to be approximately as follows :- The central line consists of three unpaired plates, of which the anterior is much the largest, and much larger than any other plate on the body; it is heart-shaped. The central and posterior are nearer to a square or oblong form. The lateral rows are each composed of four plates of irregular forms ; the hindmost is nearly triangular and approaches near to its fellow on the other side of the body. I am not sure that iu some specimens some of these plates may not coalesce or be broken up into more than one. The smaller plates generally form an irregular longitudinal line of four on each side, starting from between the first large central plate and the eyes and passing between the central and lateral lines of large plates. The edge of the boclg.-There are some of the small plates, and one or two of the nature of the large plates, on the actual lateral and posterior edge of the body. The ventral surface (fig. 2).-The epimera are arranged in two groups on each side of the body, those of the first and second leg being anchylosed together; and the same taking place with those of the third and fourth, but a strip of soft cuticle intervening between those of tbe second and -third. Between the epimera of the first pair of legs, but not attached to them, is a plate formed of the fused sternal plate and maxillary lip ; this plate does not extend as far back as the epimera do. The body forms a lateral, almost square, projection between the epimera of the second and third legs. There are, on the ventral surface, two pairs of irregular-shaped plates near the posterior margin, of the nature of the larger dorsal plates ; four or five pairs of the nature of the small dorsal plates ; the anal plate (if that be its proper name); a small plate just anterior to the anal and like it in form, but turned in the reverse direction; and, finally, the plates surrounding the genital opening. The anal plate (fig. 10) has a round central opening, which is the exterior orifice of the excretory organ and is closed by two soft labia. This opening is surrounded by a ring of chitin rather denser than the remainder of the plate, which ring is pierced by a regular row of very fine pores. A little further out is a concentric ring of rather larger pores, and tbe rest of the plate has pores similar to those in the epimera &c. The sclerites surrounding the genital aperture are similar in both sexes, and consist of a small, anterior, median, almost triangular plate of rough chitin with the point directed forward, and two , paired lateral plates, the shape of which will be best gathered from the drawing; these nearly touch posteriorly, but are further apart anteriorly. Two pairs of the so-called genital suckers are situated between these lateral plates, and one pair of larger ones behind and PEOC. ZOOL. Soc-1895, No. XII. 12 |