NILS ROSÉN, STUDIES ON THE PLECTOGNATHS. 13 



than Allen's paper about the Loricati, wliich, on the otlier 

 hand, contains a very excellent and thorougli account of the 

 whole blood vascular system of the above-mentioned group. 

 From the ventral end of the first efferent branchial artery 

 arteria hyoidea (Fig. 9, h) originates. Following Allis' views 

 on the arterial branchial system this artery ought certainly 

 to be called a. mandibularis as it seems to correspond to 

 greatest part to the afferent vessel of the mandibular arch. 

 Allis adopts the hypothesis of Dohrn and Maurer that 

 the pseudobranch represents the reduced gill of the mandi- 

 bular arch, thus not being homologous to the hyoidean gill 

 of the Elasmobranchs as often believed. Following this opi- 

 nion both the afferent and efferent vessels of the hyoidean 

 arch have disappeared in Spheroides. It is, however, imposs- 

 ible to decide with perfectly certainty whether a part of 

 the efferent hyoidean vessel has left any contribution to the 

 forming of the pseudobranchial afferent vessel without a 

 study of the development of this vessel. Immediately after 

 the hyoidean artery has passed from the medial side of the 

 hyoidean arch to the lateral through a foramen a small, but 

 lon g artery is sent off, running towards the jaws. It seems 

 to correspond to the arteria mandihularis of Allén (Fig. 9 

 mand.): A little further dorsally the hyoidean artery divides 

 into two vessels, one of which (Fig. 9 *) I have not been 

 able to follow with certainty in my non-injected slides. The 

 other supplies, after a parting in two branches, the pseudo- 

 branch (Fig. 9). The blood from the pseudobranch is collec- 

 ted by a vessel forming arteria ophthalmica (Fig. 9 oph.) which 

 with Allis' terminology corresponds to the efferent mandi- 

 bular vessel. From the ventral ends of the second pair of 

 efferent branchial arteries a small artery arises on each side, 

 uniting ventrally to the truncus arteriosus. These form arte- 

 ria veniralis (Fig, 9, ventr., Pl. II, red vessel) which runs 

 between the &ternoh3^oidean muscles, and along the ventral 

 wall of the pericardial cavity, then describes an abrupt curve 

 towards the ventral side of the body between the anterior 

 parts of the longitudinal ventral muscles and the lateral 

 veins. Having reached the ventral side it continues caudally 

 along the median line, embedded in the muscles and sending 

 off a great number of branches to the muscles and the inte- 

 gument. I ends a little cranially to the cloacal opening. 



