ANNULATA. 



[ 44 ] 



ANNULATA. 



in each space a distinct nucleus (PI. 40. 

 fig. 16 c), and in very yo^n^ animals the 

 clear spaces are distinct cells, distinguishable 

 from the surrounding cells by their size and 

 containing numerous clear vesicles as well as 

 a nucleus. The smaller cells contain a 

 nucleus and numerous nuclear granules. 

 Beneath this cellular layer are numerous 

 large fat-cells, pigment cells and connective 

 tissue, the latter consisting of a transparent, 

 homogeneous, semisolid mass. A layer of 

 fine but firm fibres, crossing each obliquely, 

 is said to be sometimes met with beneath the 

 epidermis and forming a corium or true 

 skin. 



In the Turbellaria, the outermost cuta- 

 neous layer consists of ciliated epithelium. 

 The opalescent and often beautifully colom-ed 

 skin of many of the Annulata does not gene- 

 rally owe its tints to distinct pigment, but to 

 iridescence produced by the fibres. 



The rings of the body are usually furnished 

 with bristles or hahs, sometimes an-anged in 

 tufts, at others covering the greater part of 

 the surface of the body. Sometimes foli- 

 aceous appendages cover the body like scales. 

 Most of the Annulata are covered with a 

 kind of mucus, secreted by the cutaneous 

 glands ; some live in leathery tubes or 

 sheaths, in others a case is made by the con- 

 solidation of the secretion from some part of 

 the skin with fragments of shells, grains of 

 sand, &c. ; in others, again, the calcareous 

 tubes appear to be wholly secreted by a 

 portion of the cutaneous surface. 



The muscular system is usually well deve- 

 loped. The muscular fibres are in some 

 arranged in three layers, an outer consisting 

 of annular, an inner of longitudinal, and an 

 intermediate of oblique fibres; in others 

 there is an outer layer of oblique fibres, an 

 inner of longitudinal, with annular fibres at 

 the two ends of the body. The muscular 

 fibres consist of cylinders, the transverse 

 section of which is rounded (PI. 40. fig. \7a), 

 flattened or incurvated (PI. 40. fig. \7 b). 

 They are covered externally by a delicate 

 sheath or sarcolemma (PI. 40. fig. IS b). The 

 cylinders themselves consist of a clear, ho- 

 mogeneous, cortical substance (PI. 40. fig. 18a), 

 and an internal cavity (c), the latter being 

 filled with a finely granular substance, in 

 which scattered nuclei are imbedded (PI. 40. 

 fig. 17 c). At the two ends of the body, the 

 muscular fibres branch dichotomously (PI. 40. 

 fig. 19 c). The fibres are usually smooth, 

 but sometimes longitudinally or transversely 

 striated -, this appearance arising either from 



folds in the sarcolemma or proper sheath, or 

 from the granules being arranged in linear 

 series. 



In the Turbellaria, the muscular system is 

 but slightly developed, the tissue beneath the 

 skin consisting of globular masses resembling 

 the general parenchyma of the body ; and in 

 this, peculiar cellular bodies are often im- 

 bedded, resembling the urticating organs of 

 the polypes. These enclose six, eight, or 

 more rod-shaped bodies, which are sometimes 

 parallel with each other, sometimes somewhat 

 spirally curved. The cell-membrane of these 

 bodies subsequently disappears, and they 

 frequently project beyond the skin. Ley dig 

 figures similar rod-shaped bodies as occvu-ring 

 in the nuclei of the fat-cells situated beneath 

 the skin. 



In many of the Annulata, the muscular 

 fibres are grouped into distinct bundles, 

 serving to move the bristles, parts of the 

 mouth, &c. 



Beneath the skin at the ends of, or all 

 over the body, a number of peculiar glands 

 exist ; these consist at the closed end of a 

 nucleated cell (PI. 40. fig. 19 b, d), and a 

 long, somewhat coiled duct opening at the 

 surface of the body. 



The nervous system consists of a longitu- 

 dinal, single or double series of ventral 

 ganglia, connected by longitudinal cords ; 

 the uppermost ganglion lies above the oeso- 

 phagus, and the two cords which connect 

 it with the second ganglion encircle this 

 organ. In some, the ventral ganglia are 

 absent. 



The uppermost ganglion is enveloped in a 

 neurilemma consisting of longitudinal and 

 transverse fibres, and not unfrequently pecu- 

 liar pigment cells. The cords and filaments 

 are composed of extremel)^ delicate primitive 

 fibres, between which, in the ganglia, gan- 

 glion-globules are situated. The filaments 

 distributed to the body arise principally from 

 the ganglia. 



Many of the Annulata are furnished with 

 eyes; these are usually denoted by the brown, 

 black or red spots seen upon various parts of 

 the body. It is a disputed point whether 

 all these rejiresent ti*ue eyes or not; but 

 M. Quatrefages has described a lens, trans- 

 parent cornea and vitreous humour in some 

 of them, and he has no doubt that the red 

 points found at the sides of each ring in 

 several species of Nais are true eyes. 



In some Annulata, no distinct head is 

 present ; in others this is distinguishable by 

 its form, and is furnished with eyes and one 



