and the Endopliragmal System o/Astacus. 73 



backward we find the continually thickening flexor muscle of 

 the abdomen with which we are all familiar. 



Owing to the comparatively undifferentiated character of 

 the long muscle-bands of Apus^ treated as a primitive crus- 

 tacean, it is ])robable that no part of the band contracted 

 without leading to a partial contraction of the rest. We 

 accordingly find the ventral curve of the body much more pro- 

 nounced in the Brachyura than in the Macrura. In the former, 

 while there seems to have been no lateral compression, the 

 longitudinal compression of the thorax has been much greater 

 than in the latter, and the maxillipedes are far more pro- 

 nounced as mouth-parts than they are in Astacus. 



There are other points in the anatomy of Astacus which 

 can be shown to have been modified from a more primitive 

 condition, such as we find in Ajjus. But we must pass over 

 these for the j)resent and conclude with a few descriptive notes 

 on the sections of Ajoics glacialis given in the drawings. 



EXPLANATION OF PLATE V. 



Descriptio7i of Sections. 



These are all from camera lucida outlines. The detail of the inner 

 organization is somewhat simplitied, e. y., the sections through the branch- 

 ings of the liver and the genital glands are omitted in 6. 



Fiy. \. Sagittal section through the right side of a specimen ot Apus 

 (jtaeialis, Kriiyer. 



cKj, the salivary gland, attached to the body-wall b}' muscle- 

 bands, like a typical acicular gland. This may be followed 

 through the sections till it opens in a transverse groove shown 

 in Section 7. 



S(/, the sheU-gland, which is here seen running out laterally 

 towards the dorsal parapodium of the 2nd maxilla (?«"). 



L, labrum, or large upper lip. 



m, mandible. 



I, metastoma or underlip, which is here seen projecting 

 laterally. 



m' , in" , 1st and 2nd maxillae. 



1, 2, 3, first three trunk-limbs which, in Astacus, were forced 

 forward towards the mouth as maxillipedes. 



4, the 4th limb which, in Astaais, becomes the forceps. 

 Fii/. 2. The same, six sections further on. 



a, the apodeme in front of the mandible, probably due to the 

 forcing of the latter forward. 



sp, the sternal plate, which first cornea in view as a strongly 

 curved vinss. This is of interest in connexion with my at- 

 tempted deduction of Apits from an Anni'lid with the first 

 five segments bent round to form the new head. The curve is 

 very pronounced in Sections 2, 3, 4, and partly in 6. In Sections 

 6 and 7, which lie nearer the middle line, we liavo onh'fhe pos- 

 terior portions of the sternal plate, which naturally lie straiglit. 



b is the large fold behind the underlip, due, I think, to the 

 forcing of the lip backward. 



