Consider the following statements with reference to Hexactinellida:
A. They are entirely marine and are found in shallow water.
B. Spongin is lacking.
C. Cellular dermal epithelium is well marked.
The correct statements/statement are/is:

Question

Consider the following statements with reference to Hexactinellida: 
A. They are entirely marine and are found in shallow water. 
B. Spongin is lacking. 
C. Cellular dermal epithelium is well marked. 
The correct statements/statement are/is:

Accepted Answer

The correct option is 2: only B

Hexactinellida, commonly known as Glass Sponges, are a fascinating class within the phylum Porifera, distinguished by unique anatomical and skeletal features. The correct statement is B, "Spongin is lacking." The skeleton of Hexactinellida is almost exclusively composed of siliceous spicules, specifically those that are characteristically hexactinal (six-rayed), which gives them their crystalline, glass-like structure and their class name. Unlike the Demospongiae, which form a complex fibrous network using the protein **spongin** alongside silica or sometimes alone, Hexactinellida entirely lack this proteinaceous skeletal component, relying solely on the intricate framework formed by their silica spicules. Furthermore, the body plan of these sponges is unique in that much of their tissue is a syncytium—a continuous, giant, multinucleated cytoplasm rather than distinct, separate cells—a feature that directly relates to why statements A and C are incorrect.

The incorrect option is 1: only A

Statement A is "They are entirely marine and are found in shallow water." While the first part of this statement is correct—Hexactinellida are indeed **entirely marine**—the second part regarding their habitat is largely inaccurate for the class as a whole. Glass Sponges are overwhelmingly **deep-sea or abyssal** organisms, with the vast majority of species found at depths greater than 200 meters. Their distribution is a defining characteristic, differentiating them from other sponge classes often found in littoral or shallow shelf environments. Although some species may inhabit shallower, colder waters (like in the Antarctic), their principal ecological niche remains the deep ocean, making the general description "found in shallow water" misleading and incorrect in this context.

The incorrect option is 3: only C

Statement C is "Cellular dermal epithelium is well marked." This is factually incorrect because it completely contradicts the defining histological organization of the Hexactinellida. The dermal layer (pinacoderm) in most sponges is a distinct, cellular layer made of pinacocytes. However, Glass Sponges are notable for having a **syncytial dermal membrane** (or trabecular net), which is essentially a continuous, multinucleated sheet of cytoplasm without the separate cell boundaries typical of a true cellular epithelium. This syncytial structure is a defining, fundamental characteristic of the Hexactinellida and means that a "well marked cellular dermal epithelium" is precisely what is absent in this class.

The incorrect option is 4: A, B and C

This option asserts that all three statements are correct, which is untrue because we have established that statements A and C contain factual inaccuracies about the class Hexactinellida. Statement A fails to accurately describe the typical distribution of Hexactinellida, as they are deep-sea organisms, not shallow-water ones. Statement C is fundamentally incorrect because these sponges possess a **syncytial dermal membrane** rather than a clearly demarcated *cellular* dermal epithelium. Since two out of the three statements are incorrect, the option claiming all three are correct must be false.