. Elements of Comparative Anatomy. 168 COMPAEATIVE ANATOMY. one anotlier. New longitudinal trunks are formed from tlie vessels distributed to the enteron. While in these forms a complex system is produced by the combination of the primitive median trunks with a system of canals formed from lacunae of the coelom, the whole vascular system may be made simpler by the disappearance of these median trunks. This is the case in Nephelis, where there is a wide median sinus^ and two lateral vessels. This form of vascular apparatus^ developed out of a lacunar system, is limited to the Hiru- dinea, for i
. Elements of Comparative Anatomy. 168 COMPAEATIVE ANATOMY. one anotlier. New longitudinal trunks are formed from tlie vessels distributed to the enteron. While in these forms a complex system is produced by the combination of the primitive median trunks with a system of canals formed from lacunae of the coelom, the whole vascular system may be made simpler by the disappearance of these median trunks. This is the case in Nephelis, where there is a wide median sinus^ and two lateral vessels. This form of vascular apparatus^ developed out of a lacunar system, is limited to the Hiru- dinea, for in the Annelides the vascular system is almost always shut off from the coelom. Where it is not so, we have to do, not with further development, as is the case in the differentia- tion of the coelom of the Hirudinea, but with degeneration. The dorsal vessel lies, as a rule, immediately upon the enteric canal, and often appears to be embedded in the same glandular layer. In addi- tion to anterior and posterior connecting vessels, there are lateral vessels which correspond with the metameres. They are divided into those which directly surround the enteron, and form a capil- lary network, often a highly-ramified one, in its walls (visceral vessels), and into those which pro- ject into the coelom, and run either along its walls or its appendages (parietal vessels). In the Scoleina the arrangement is generally the same throughout the whole body. In many cases the transverse vessels, as well as the dorsal longitu- dinal trunks, are pulsatile, and one or more pairs are considerably widened (Fig. 77, c). In this differentiation of a portion of the vascular system we see the beginning of the development of a central organ of the circulation—a heart. The ventral vessel is very seldom contractile. Fresh complications in structure are due to the develop- ment of fine vascular networks, such as, in Lum- bricus for example, are distributed as capillaries through the body. Branchiobdella is
Size: 810px × 3086px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bo, bookcentury1800, bookdecade1870, bookpublisherlondonmacmillan
