砂壳纤毛虫（学名：Tintinnina），又名有钟纤毛虫或丁丁虫，泛指隶属于纤毛虫门旋毛纲之下丁丁虫亚目的物种。丁丁虫亚目旧属旋毛纲，现时被重新分类，成为了寡毛纲Choreotrichida目之下的五个亚目之一。本分类单元包括有种数在1000种左右，大多数在海洋中营浮游生活，属微型浮游动物（microzooplankton），主要摄食浮游硅藻和鞭毛虫，是水体微食物环的重要组成部分，在水体中的能量流动和物质循环中占重要地位。
砂壳纤毛虫的虫体生活在壳中，其外壳是其独特的标志。有部分文献依从日语文献，称呼这种外壳为胴甲（英语：Lorica (biology)）（）。这些外壳大多呈壶形、花瓶形或管形，主要由蛋白质及小量矿物质构成，都有一个开口，有的在反口面还有一个较小的开口，虫体有一个柄固着在壳的后端，通过身体的伸缩可以将身体伸出和缩进壳内；在环境不利时，虫体会离开壳。
形状和大小类似于丁丁虫的胴甲的化石，被称为Calpionellids（长约50到200微米，已于中生代灭绝），最早出现在奥陶纪，但由方解石形成，并且由于没有已知的丁丁虫会生成含方解石成分的纤毛状外壳，因此它们不太可能是丁丁虫的化石——甚至可能根本与纤毛虫完全无关。能够与现生丁丁虫相关连的化石记录（例如：黏着在一起的胴甲化石）出现于侏罗纪时期，但数量并不多，直到白垩纪才开始大量出现。xTintinnids are an important part of the fossil record because of the rarity with which most other ciliates become preserved under the conditions of the marine environment. The loricae of some tintinnids are easily preserved, giving them a relatively good fossil record.
不同种的砂壳纤毛虫的壳或胴甲的长度和口径变化很大。壳长最大的记录是巨拟网纹虫（）壳为750 μm(Marshall, 1969)，最小的壳为极小领细壳虫（），壳长13 μm(Hada, 1970)。同一个种中，不同海域不同个体间的长度差可达300 μm。例如，巨拟网纹虫的最大记录750 μm (Marshall, 1969)，但是大多为337-576 μm(Campbell, 1942)。同一种中不同个体壳的大小可能是年龄的标志，小壳是幼体，大壳则是成熟的个体(Gold and Morales, 1976b)。
They were among the many planktonic microorganisms featured in Ernst's 恩斯特·海克尔's classic work popularizing the beauty of the natural world "Art forms in Nature" (自然界的艺术形态).
如同其他原生生物，砂壳纤毛虫属于一种复杂的单细胞真核生物。它们是丁丁是异养水生生物（heterotrophic aquatic organisms）， 以能进行光合作用的藻类和细菌为食，均为 体长介乎20到200微米的微型浮游生物。Tintinnids are found in marine and freshwaters. However, they are most common in salt water and are usually present in concentrations of about 100 a liter but can reach abundances of several thousand per 升. Characteristics of their lorica, or shells, are classically used to distinguish the roughly 1000 species described. However, in recent years application of histological and molecular techniques have led to many taxonomic revisions.
Many species appear to have wide distributions (for example from the 切萨皮克湾 to 新喀里多尼亚) while others are restricted to certain areas, such as arctic waters or coastal seas. Nonetheless, in any given locale dozens of species can be found. Like other members of the microzooplankton (such as oligotrich（英语：oligotrich） ciliates, heterotrophic 双鞭毛虫门s, radiolarian（英语：radiolarian）s, etc.), tintinnids are a vital link in aquatic food chains as they are the '食草动物s' of the plankton. They feed on 浮游植物 (algae and cyanobacteria) and in turn act as food for larger organisms such as 桡脚类s (small 甲壳亚门s) and larval fish.
The color image on the right is a specimen of from the Bay of 滨海自由城 in the Mediterranean Sea. The hair-like projections pointing out of the top of the shell are the cilia of the cell. The cilia generate a water flow across the mouth of the cell, bringing food into contact and move the tintinnid. Their swimming pattern is rather 'jumpy'- or dancing- they are part of the 'choreotrichs' which means dancing hairs from their swimming behaviour and cilia.
丁丁虫亚目包括下列各个科：
Life history stages of a tintinnid, Eutintinnus inquilinus. In ciliates reproduction (B&C) is divorced from sexual recombination (D&E).
Living cells from the Bay of 滨海自由城 (N.W. 地中海)
Tintinnids of the California Current Ecosystem
Amundsen Sea (Antarctica) Tintinnids
Images of tintinnids and other microplankton found in the Chukchi Sea in the Arctic
Tintinnids of the Thau Lagoon (Sète, France)
Tintinnids from the estuarine region of the Ganges River in India
