6 ARKIV FÖR ZOOLOGI. BAND 8. N:0 18. 



of m. rectus. (Fig. 4). A recto-muscle is present. Its form has 

 been influenced very much by the development of an air-sac. 

 The subcutaneous tissue whicb lies between the skin and the 

 airsac (ef. section 2 and 3 of this series of studies) 2 has sepa- 

 rated this musele in two distinct layers, one exteriör and 

 one interiör. The interiör attachés behind to the anal fin, 

 cranially to the pectoral arch. A small lateral portion is 

 distinct and is inserted into the free tip of the postclavicle. 

 Cranially this part of the rectus is in close relation to m. 

 obliquus inferior. The muscles of the both sides are widely 

 separated from each other. The air-sac is situated between 

 them. As I have previously (Bd 7, N:r 30 of this journal) 

 described in details, the air-sac has coalesced with the ven- 

 tral body-wall. The lateral parts of the air-sac separate the 

 interiör part of m. rectus from its exteriör portion. This 

 portion represents the original medial part, the interiör the 

 lateral one. The medial part has become enlarged covering 

 the whole ventral surface of the body. One part is attached 

 behind to the anal fin. With it is j oined a portion that 

 originates from the vertebral column. The musele spreads 

 out över the ventral surface of the body. Another portion 

 forms a continuation of the caudal part of m. obliquus injerior 

 and spreads out in the same manner as the portion just 

 mentioned (Fig. 5). The fibres continue anteriorly on the 

 ventral surface of the head. Some fibres form anteriorly on 

 each side a musele that attachés to the ventral tip of the 

 pectoral arch. Along them run the anterior parts of the 

 lateral veins 2 and between them the ventral artery. As the 

 Fig. 4, 5 and 17 show, this exteriör portion of m. rectus 

 covers the rest of this musele the so called interiör portion. 

 The subcutaneous tissue between the air-sac and the 

 body wall being very highly developed, the exteriör rectus 

 musele becomes very distinct and closely connected with the 

 skin. The musele gets by this arrangement the characteri- 

 stics of a skin-muscle. Its true nature is, however, quite 

 evident. The interiör rectus musele is distinctly metamerical, 

 the exteriör indistinctly so. Outside of the exteriör rectus 

 musele is a thin layer of muscles with transverse fibres. This 

 musele has a great extension, covering the whole body, in- 



1 Arkiv f. Zoologi. Bd 7, N:r 30 and Bd 8. N:r 10. 



2 These studies. Section 1. (Arkiv f. Zoologi. Bd. 7. N.:r 25.) 



