io66 



THE NATURE BOOK 



found to be missing, yet the negative 

 truth is aknost of as much \'ahie evi- 

 dentially as the positi:^ demonstration. 

 Very frequently, however, these hnks are 

 not demonstrated by the completed ani- 

 mal form, but are to be traced in the 

 embryonic stage. 



The tribe of the Sea Slugs, or Xudi- 

 branclis. in the embryonic stage are pro- 

 \ided with a unispiral shell, whilst the 

 Li mac i dec. or Land Slugs, possess a shelly 

 plate which is embedded in that part of the 

 animal termed the " shield," or " mantle." 



The position of the " shield " is either 

 close behind the head, spreading back- 

 ward over the body in oval shape, sug 

 gestive of a mantle over the shoulders, 

 or at the rear of the animal, as is the case 

 in the genus Testacella, or Snail Slugs. 



This apparent variation in the position 

 of the "shield" is, however, due to an 

 abnormal development of the neck of the 

 Slug in advance of the " shield." The 

 Testacella hold an intermediate position 

 between the Helix or Snail family that 

 hve in shcUs, showing them externally, 

 and the Slug family, that only possess 

 interior shells. The shell of the Testacella 

 tribe is a truly formed one, often exceed- 

 ingly beautiful and deUcate in construc- 

 tion, and is shown externally, although 

 the size in respect of the animal is certainly 

 disproportionate. There is, however, no 

 pretence of it being a " house of refuge " 

 as with the Snail ; it appears to be merely 

 a protecti\'e plate that is always asso- 

 ciated with the " shield " that overlies 

 the vital organs of the animal, such as 

 the pulmonary chamber, heart, etc. The 

 noticeable respiratory orifice seen as a 

 round hole on the border of the " shield " 

 on the right side of the animal lies beneath 

 the shell of the Testacellidce. 



The tribe of Snail Slugs, carrying their 

 shells externally to the rear, are sub- 

 divided into three fixed species : (i) Tes- 

 tacella haliotidea, (2) T. scutulum, (3) T. 

 maiigei. 



The first-named is a pale yellow slug 

 about three inches long, found throughout 

 England in estabUshed locaUties. The 

 " sliield " is nearly hidden by the ear- 

 shaped shell, which is flattened rather 

 than convex in form. The illustration * 



* Photos taken from Natural History Department, 

 British Museum, Cromwell Road, South Kensington. 



shows the shell apart from the dried 

 animal, that is, of course, shrunk in bulk ; 

 but the photograph gives practically the 

 natural size of both shell and slug, al- 

 though not in the extended form. 



T. scutulum is a darker-coloured slug, 

 yellow or brown, of the same length, and 

 somewhat similar shell. T. maugei is 

 larger, about four inches long, of a dark- 

 brown colour, with a much larger shell, 

 more cylindric in shape. 



This family, as many others of the 

 Slugs, only go out after food at night, 

 remaining below the earth surface during 

 the day. They are carnivorous feeders, 

 and attack conveniently sized earth- 

 worms. 



Having marked their worm as satis- 

 factory when encountered on the nightly 

 prowl, the slug draws in the tentacles 

 and dilates the mouth, pounces suddenly 

 on the worm and proceeds to draw it in. 

 This is a very leisurely performance, and 

 on such occasions the expansive capacity 

 of an invertebrate comes into full play 

 of possibiUty. It is stated that an average 

 hfe for these slugs is as long as five or 

 six years. 



In respect of the Land Molluscs gener- 

 ally, it has been noted that variations of 

 colouring are more marked in the southern 

 and western counties, when the type 

 itself becomes rare. Mild climates pro- 

 duce stronger-coloured forms ; colder 

 chmates the dull-coloured typical forms. 

 Variations appear to be also produced by 

 the geological conditions of the habitat 

 and by differences of food. 



The eggs of the Testacella are deposited 

 at considerable depth in the ground, in 

 galleries bored by the animals ; they are 

 coated with a calcareous shell that bursts 

 when brought into contact with warmth. 

 The young slugs hatch after ten to thirty- 

 five days, according to the species, and 

 are full grown in eighteen months, Uving 

 five to six years. The south-west counties 

 of England seem to be the favourite locali- 

 ties for this slug. 



The two large classes of the Limacida 

 and Ariomdcc are each subdivided into 

 what may be termed typically specialised 

 Slugs. 



In respect of shell formation, the 

 Limacidcc come nearest to the Testacellidce 

 as possessing definitely formed shells, 



