Ilointsur Tiilnsjuiincdliij .[niitlitls. ,'J 



fi;ivc hut a sli^^Iit imlicatioii of tlifir im[)ijitaiicc auiuiii^-.st llic 

 Invertebrates of our sliure.s. 



Again, it is long since the connjlexity of tlic locomotive 

 a))j»entlages — even ot" the most simple amongst the marine 

 annelids — has excited the admiration ot" observers. Every 

 foot is furnished with two tutts of bristles, dorsal and ventral, 

 or with bristles and hooks, or with hooks alone. Moreover, 

 these organs are each capable, by their muscular apparatus, 

 ot" |)ushing the aiuielid out ot" its tube, of fixing it in it, or 

 of retracting it. For, as Dr. Thomas Williams* formerly 

 pointed out, even in those with calcareous tubes the inte- 

 rior is not so smooth as to be slippery, or so hard as to 

 render it impossible to fix bristles or hooks in its progress 

 outward or inward. " Computing," as he says, " the pushing 

 force which each seta is capable of exerting, and multiplying 

 this amount by the number of setai in each foot, and this 

 again by the number of feet with which the annelid is pro- 

 vided, a conception may be formed of the aggregate of mecha- 

 nical power with which the animal executes its ' march 

 forwards.' A similar calculation applied to the hooks wil'i 

 give a correspondingly prodigious resultant of power for 

 retreat." 



The presence or absence of a special home or habitation in 

 the marine annelids was, and still is, by some supposed to 

 distinguish the tw^o great groups from each other, viz., the 

 Erratic (or Annelida Errantia) from the Sedentary (Annelida 

 Sedentaria) ; but in a natural classification this does not 

 altogether hold true. For example, the iSyllidians are really 

 errant annelids ; yet many form trans[)arent tubes, and others 

 pass their lives in the interior of sponges. Similar habits, 

 with strong tubes, occur in the Nereids and Euuicids. Some 

 annelids furnished with tubes, again, are by no means seden- 

 tar}', such as Lagis and Ampliictene. Moreover, it is not 

 always the most delicate forms that have such a protection : 

 thus the Phyllodicidte are free, while certain Lepidotioti occur 

 only in the tubes of other annelids. 



Limited as the group of annelids appears to be in the great 

 world of marine animals, it is found that their habitations 

 range themselves under at least eleven heads. 



Thus some, such as Spinther and Euphrosyne, dwell on 

 sponges, which serve them as food, and they browse on these 

 spicular pastures after tlie manner of tlie sea-lemons amongst 

 the Nudibranchs. Others affect the interior of the sjjonge, 

 like the Syllidians, amongst which the remarkably branclied 

 Si/Uis ramosa of the ' Challenger ' stands out conspicuously. 

 * Report Brit. Aseoc. 1651, p. 204. 



1* 



