The Morphology of Cosmobia 391 



by lack of space for a full development. These specimens are therefore 

 obtainable of a size large enough for convenient dissection and thus the 

 circulatory system has been well described in a number of cases. The 

 conditions of the heart and the larger blood-vessels in six eases, including 

 the present one, are for convenient comparison reduced to diagrams of 

 similar construction [Fig. 19]. These have been taken from the original 

 description in each case. In some cases the description was verbal 

 only, in others the text was accompanied by one or more figures ; usually, 

 however, those latter were drawn as if viewed from the side, thus requir- 

 ing translation into the diagrammatic form here adopted. Mayor^ 

 however, makes use of a sectional diagram similar to mine, but includes 

 rather more parts than are here necessary. In all cases excepting one 

 (VI), pulmonary as well as aortic arches are employed in forming the 

 aortse, ichich, if all developed, would form in such a diagram a perfectly 

 symmetrical figure with arches in the form of a rhomboid. The actual 

 cases show, however, that one out of the four always fails to form a com- 

 plete connecting arch, and persists only at its cardiac end, as a variously 

 used adjunct to the rest, while the remaining portion, which would 

 naturally span the interval between a heart and an aorta, disappears. 

 Since in the diagram each monster is shown with the imperfect and per- 

 fect sides and the A and B components in the same relative positions, 

 an exact comparison of the cases is easily made ; and in this way it may 

 be seen that the missing arch may be any one of the four. Thus in the 

 first four of the diagrams, taken from as many different authors, each of 

 the four possibilities is shown; JSTo. V is much like No. Ill ; Xo. VI shows 

 a quite different result from any of the others. 



The relations of pulmonary arteries, and of the carotid and subclavian 

 trunks, show also some variation, depending on the identity of the per- 

 sisting arches. For instance, the pulmonary arteries seem always to 

 arise, as is natural, from a right ventricle through the pulmonary arch, 

 and in cases where this vessel fails as a complete arch (as on the perfect 

 side of ISTos. I and IV, or on the imperfect side of Nos. II and V) a stump 

 still persists which gives rise to the vessels, after which it usually anasto- 

 moses with the arch that persists. ISTo pulmonary system is given in 'No. 

 VI, since the paper is silent concerning these vessels. In the case of the 

 carotids the most typical arrangement is undoubtedly that shown in No. 

 I, where a pair comes from each aortic arch; yet, while the arch itself 

 passes over into the dorsal aorta of a single component, the two caro- 

 tids that proceed from it supply the two sides of a neck that faces either 



