SPHENELLA. 



[ 591 ] SPIRAL STRUCTURES. 



Brit. Jung. pi. 33, and Suppl. pi. 2 ; Ekart, 

 Syn. Jung. pi. 6. figs. 43 & 48. 



SPHENELLA, Kiitz. A genus of Dia- 

 tomacese. 



This genus appears to consist of the 

 detached frustules of Gomphonema. 



Kiitzing describes seven species. 



BIBL. Kiitzing, Bacill. 83, and Sp. Alg. 



SPHINCTOCYSTIS, Hass. (Cymato- 

 pleura, Sm.). A genus of Diatomaceae. 



Char. Frustules free, single ; in front 

 view linear, with undulate margins ; valves 

 oblong or elliptical, sometimes constricted in 

 the middle. Aquatic. 



Valves with coarse, transverse, or nearly 

 transverse, rounded elevations appearing as 

 dark bands, an interrupted median hue, coarse 

 lateral dots and transverse striae, but neither 

 alas nor nodules. 



Five British species. 



S. solea (PI. 12. fig. 23). Valves linear- 

 elliptic, narrowed on each side towards the 

 middle, transverse striae evident ; extreme 

 length 1-216". 



Undulations six. Common. 



/3. Much shorter, undulations four, ends 

 apiculate. 



S. elliptica(P\. 12. fig. 24). Valves broadly 

 elliptic or elliptic-oblong, striae obscure, 

 undulations four or five; length 1-280". 



Common. 



S. Hibernica. Valves broadly elliptic, 

 acuminate, undulations three ; length 1-250". 



BIBL. Hassall, Brit. Freshw. Algce, 436; 

 Smith, Brit. Diat. i. 36 ; Kiitzing, Sp. Alg. 



SPICULA (plural of spiculum), In some 

 of the Invertebrata, firmness is given to the 

 body by a rudimentary external skeleton 

 consisting of a number of curiously shaped 

 microscopic bodies, many of which are of 

 a needle-like form, often containing a cavity, 

 and denominated spicula. They are met with 

 in endless variety of form in sponges (PL 37, 

 the lettered objects), where they consist of 

 silex. They also occur in the Echinodermata 

 (PI. 37. figs. 1 h, i, k, I and 19 a, b, c), the 

 Foraminifera (PI. 18. fig. 24), and in the 

 Mollusca, in these instances being calcareous. 



There can scarcely be doubt that spicula 

 are homologous with the elements of shell ; 

 but little or nothing is known of their deve- 

 lopment. 



Spicula form very interesting microscopic 

 objects, on account of their remarkable 

 forms. 



To prepare them, the animal substance 

 in which thev are contained should be 



boiled with nitric acid if they are com- 

 posed of silex, and with dilute solution of 

 potash if they consist of lime-salts. They 

 may be preserved by mounting in Canada 

 balsam. 



They are commonly met with in sea-mud. 

 SPIDERS. SeeARACHNiDAandEpEiRA. 

 SPILOCJEA, Fr. A genus of Torulacei 

 (Coniomycetous Fungi). S. Pomi occurs 

 upon apples, in contiguous effused patches, 

 from which the epidermis separates in frag- 

 ments, exposing the simple globular spores, 

 adherent to each other and to the matrix. 



BIBL. Berk. Brit. Flor. ii. pt. 2. p. 360 ; 

 Fries, Summa Veget. p. 482. 



SPINES, OF ANIMALS. These are pro- 

 perly stout rigid and pointed processes of 

 the integument, formed externally by the 

 epidermis, and internally of a portion of the 

 cutis or corresponding structure; but the 

 term is frequently applied to stout rigid and 

 pointed processes of the epidermis only. 



See HAIRS, and the notices of the struc- 

 ture of the integument under the heads of 

 the various classes. 



SPIRACLES or STIGMATA of animals. 

 The external orifices of the tracheae of insects 

 and Arachnida. The respiratory tubes of 

 these animals have no communication with 

 the mouth, but terminate externally in ori- 

 fices situated upon the surface of the thorax 

 or abdomen. These are mostly rounded or 

 eUiptical(P1.28. figs. 3,7, 8, & 9 a), sometimes 

 in the form of small clefts, and are often 

 furnished with a kind of moveable valve, or 

 bounded by a thickened rim ; sometimes a 

 sieve-like structure (PI. 27. fig. 34) prevents 

 the admission of foreign bodies, or they are 

 surrounded by hairs or scales, effecting the 

 same purpose. 



They are often situated at the lateral and 

 upper portions of the abdomen, at the pos- 

 terior, lateral and upper part of the thorax, 

 &c. 



See ARACHNIDA, INSECTS, and the heads 

 of the genera. 



SPIRAL STRUCTURES, OF PLANTS. 

 Among the most elegant of the microscopic 

 objects furnished by the Vegetable Kingdom 

 are the various forms of the secondary depo- 

 sits upon the walls of cells, vessels and ducts, 

 &c., which present the appearance of fibres 

 coiled into perfect spirals, or of spiral fibres 

 either with the coils detached and forming 

 rings, or with the coils more or less con- 

 nected by cross-pieces, producing a reticu- 

 lated structure, 



Under the head of SECONDARY DEPOSITS 



