366 Prof. AUman on Chelura terebrans. 



side. It supports three pairs of heteromorphous appendages. 

 Those of the first pair consist each of a long basal stem termi- 

 nated by two small, one-jointed rami (figs. 1, 18), and articu- 

 lated upon the inferior edge of a small vertical plate, which is 

 placed at each side of the anterior end of the segment. The 

 second pair is articulated upon the same plate, at a point nearly 

 vertically over the origin of the first. It is a large lamellar 

 organ (fig. 1) fringed with hair, and having two fringed leaflets 

 articulated on its edge ; in its habitual position it is thrown up 

 vertically upon the back, with its surfaces directed, one inwards 

 and the other outwards. The appendages of the third pair con- 

 stitute a sort of tail by which the body is prolonged backwards ; 

 they are borne upon the posterior extremity of the segment, and 

 consist each of a very large leaf-like lamina supported on a short 

 basal joint* (figs. 1, 17) ; the margins of the lamina are ser- 

 rated, each serration bearing a minute but strong spine. 



The terminal segment of the abdomen assumes the form of a 

 small leaf-lilce lobe (figs. 1, 17) placed between the origins of the 

 two last-mentioned appendages. 



The true import of the great abdominal trunk, with its ap- 

 pendages, may now for a few moments arrest our attention. 



The probability of its being made up of three distinct elements 

 would at once be suggested by the fact of its bearing three pairs 

 of appendages, as well as by the circumstance that such view 

 would establish the normal number of abdominal rings; but 

 then the peculiar position of its two anterior pairs of append- 

 ages, one being placed vertically over the other, as well as their 

 arising from a common intermediate plate, would present itself 

 as a difficulty to this mode of viewing the subject. A careful 

 examination however will get rid of the difficulty, and enable us 

 to reconcile the apparent anomaly with the fact, that this single 

 trunk is really made up of three confluent segments, each bear- 

 ing its own pair of appendages. If the under surface of this 

 part of the abdomen be examined, it will be found marked near 

 its anterH)r extremity by two transverse grooves, terminated at 

 the sides by the lateral plates already mentioned, and plainly in- 

 dicating the existence of three originally distinct rings, of which 

 the posterior alone becomes greatly developed. The lateral 

 plates must then be viewed as the very unequally developed epi- 

 sternal pieces of the first two of these rings, so consolidated as 

 to leave no trace of their original distinctness, while the second 

 episternal piece being developed in a forward direction, and pass- 



* It is these basal joints of the two caudal appendages which Philippi 

 seems to have mistaken foi* a fifth abdominal segment, with the anus in a 

 fissure on the back. 



